I finally decided to read Dune as I have never read a book from that series. I immediately had a question. Why doesn't the Emperor control Dune (Arrakis) himself? Why does he let houses control it? As I understand, melange is the substance without which space travel is not possible. So to me it looks like the Emperor must control it to guarantee his power. How could you let your vassals (and possibly your rivals) control critical resource? Or he is merely not that powerful?
There are a couple of influences at work here.
The first, and simplest, is that the Emperor was subject to the decisions of the Landsraad, even though he sat on the High Council. The combined might of the Great Houses posed a significant threat to the Padishah Emperor, so he had to avoid angering the Landsraad enough to motivate the Great Houses to unite against him.
Remember that each of the Great Houses still had their caches of atomic weapons, and could do significant damage if they felt pressured enough to break the Great Convention. The spice melange was important enough (as you mentioned) that the Emperor seizing direct control of the only source would probably have been seen as sufficient threat by all of the Great Houses to cause them to put tremendous political pressure on him, if it didn't push them to outright rebellion. While the Sardaukar are still feared as the most deadly troops in the Galaxy (until their defeat by the Fremen), they can't be everywhere. The combined forces of the Great Houses, even without the House atomics, would have been a very serious threat.
Similarly, the Guild of Navigators would probably have looked unfavorably upon such a move by the Emperor. While their monopoly on space travel put them in a position of great strength, they would likely have been concerned that such a move on the Emperor's part would have threatened to disrupt the balance of power that they relied upon to maintain their position of superiority.
Finally, control over the planet Arrakis is a tremendously attractive carrot to dangle in front of Great Houses in order to get them to support the Emperor's plans (or to act as bait to help betray enemies who threaten to become to powerful, as was the case of Duke Leto Atreides). Whichever House was granted control of the planet would be grateful to the Emperor, and, by virtue of the tremendous income the Spice trade generated, the controlling House would also gain in strength, making them not just an ally of the Emperor, but a strong ally.
Just dangling the possibility of being granted control of Arrakis would provide the Emperor countless opportunities to apply political pressure to the Great Houses. "Support me in this, and I will remember this when I need to replace the stewardship of Arrakis" or even "if you support my opponents, it will spell the end of any chance you might have of being appointed control of Arrakis".
It does seem weird that the planet that's the only source in the known universe of the most important and valuable substance ever found is treated as a godforsaken wasteland about which little is know, or is cared to be known. The prime movers behind this are the Spacing Guild. Paul says near the end of "Dune" that the Guild could have taken control of Arrakis and the spice, lived it's glorious day and died. But, whenever the Guild's Navigators looked into the future, they saw that any attempt by them - or anyone else - to take control of the Spice would lead to a Nexus, a point in time beyond which they could no longer predict what was going to happen. So they chose instead to try to control things from behind the scenes, keeping everyone else divided, uninformed (why is there only one Imperial Ecologist on Arrakis? Why isn't the planet crawling with people trying to find out where the Spice comes from?) and dependent on their space travel monopoly. Paul tells the Emperor that the Guild permitted him to ascend his Throne. (In the movie version, a Guild representative tells the Emperor to his face that, unless he fixes things on Arrakis, he will live out the rest of his life in a pain amplifier.) The Emperor doesn't control Arrakis because the Guild didn't want him to do so.
I think it's pretty clear from the sequels that the political fallout of the emperor controlling arrakis that all of the great powers would have moved to prevent imperial control of arrakis.
Another aspect of the politics of the empire is that it's clear that whoever is on the ground controls the planet. The way the whole empire works is that the various houses are vassals of the emperor, and control their planets under grant from him. Now, the emperor can either grant Arrakis to someone closely connected to him, or to another house. If he grants it to someone closely connected to him, that person is (a) related to the imperial house; and (b) controls the single most important resource in the galaxy. That looks like a recipe for a coup. Much safer to grant the planet to someone who has no chance whatsoever of taking the imperial throne (for example, the odious Harkonnens - neither the Landsraad or the Guild would ever let them grow too powerful).
In addition, Paul Atreides is the first person to truly control Arrakis, through his control of the Fremen. The Harkonnens were the rulers under Imperial law, but in practice they seem to have done little more than rule the cities and control the legitimate spice trade.
Finally, Paul Atreides achieved the upper hand over the guild, which allowed him to attack the rest of the empire. By contrast, no other emperor even knew how to destroy the spice (which the guild depended on), and in any case would need to control the rest of their empire (requiring guild transport), while Paul did not have an empire to defend. It is also possible that had he needed to, he could have achieved limited space travel on his own, with his own sight.
Before leaving Caladan:
"I'm going to watch our screens and try to see a Guildsman."
"You won't. Not even their agents ever see a Guildsman. The Guild's as jealous of its privacy as it is of its monopoly. Don't do anything to endanger our shipping privileges, Paul."
Near the end of the first novel, when Paul's forces attack Arrakeen:
The taller of the two, though, held a hand to his left eye. As the Emperor watched, someone jostled the Guildsman's arm, the hand moved, and the eye was revealed. The man had lost one of his masking contact lenses, and the eye stared out a total blue so dark as to be almost black.
Guildsmen avoid the public and if they have to show themselves, then hide the usage of spice. In other books it is said that Navigators cease all contact with their families.
Later, when Paul confronts the Emperor:
The Guild is like a village beside a river. They need the water, but can only dip out what they require. They cannot dam the river and control it, because that focuses attention on what they take, it brings down eventual destruction.
These passages imply that the Guild is keeping its total dependance on spice a secret. They took huge spice bribes from the Fremen to keep the skies clear of satellites. But even if they had to buy large quantities of spice openly, nobody could know if it went somewhere to be sold or mysteriously disappeared, because the Guild is the only one doing space travel.
Another exchange when Paul finds Gurney:
"Gurney," he said, "are there many Guildsmen around Rabban?"
Gurney straightened, eyes narrowed. "Your question makes no . . . "
"Are there?" Paul barked.
"Arrakis is crawling with Guild agents. They're buying spice as though it were the most precious thing in the universe. Why else do you think we ventured this far into . . . "
"It is the most precious thing in the universe," Paul said. "To them."
It seems to me like Gurney doesn't realize the spice's value to the Guild. It is something Paul saw.
The Emperor didn't really know there is more to Arrakis than profits.
Remember that the Dune universe was politically a tripod:
We've a three-point civilization: the Imperial Household balanced against the Federated Great Houses of the Landsraad, and between them, the Guild with its damnable monopoly on interstellar transport.
And remember what Dune itself says about tripods:
In politics, the tripod is he most unstable of all structures.
The problem with a tripod is that you can always have two people ganging up on one. Right now, the Emperor and House Corrino compete with the other noble houses for control of the spice (and wealth, the two are interchangeable thanks to CHOAM). If the Emperor tried to seize Arrakis, the Spacing Guild would turn against him. He would be ruined because he'd never be able to transport goods or people off-world.
This is what makes the Emperor's move so risky in Dune. He was essentially doing this, via Baron Harkonnen, because he knew the Baron could be bought.
"We'll move in strengthenedby two legions of Sardaukar disguised in Harkonnen livery."
"Sardaukar!" Feyd-Rautha breathed. His mind focused on the dread Imperial troops, the killers without mercy, the soldier fanatics of the Padishah Emperor.
"You see how I trust you, Feyd," the Baron said. "No hint of this must ever reach another Great House, else the Landsraad might unite against the Imperial House and there'd be chaos."