Why would Admiral Marcus need Khan's help?

Question

In Star Trek Into Darkness, Admiral Marcus puts Khan to work for him, on projects designed to enhance the military capability of Starfleet.

For instance, Khan is a principal consultant on the construction the USS Vengeance and is intimately familiar with its design.

Why does Marcus need Khan's assistance, and how is Khan even able to give this assistance?

Khan may have a superior, generically-engineered intellect, but he is not a starship designer, he's not a phaser specialist, he's not a warp field mechanic, and so on. He is a man from the 20th Century. A cunning man, for certain. Able to outwit an opponent? Often (but not always). But I don't see how he can be so essential to Marcus' plans, which rely very much on the development of technologies fit for the 23rd Century. I realize that Khan was a quick study in TOS "Space Seed". But still.

Why would Marcus need Khan's work on Vengeance? Surely, starship designers of the time would simply listen to Marcus' requests and then follow orders to build a bigger, faster, more powerful, and more menacing ship.

I can understand if Marcus would want Khan's advice on how to manipulate the situation with the Klingons, using his experience as a one-time fairly successful despot on Earth. But starship design? Even if Khan were able to provide input, surely there are already experts working for Starfleet and Section 31 in the 23rd Century that have the know-how to realize Marcus' vision when it comes to Starfleet hardware.

Khan doesn't really have a monopoly on "savagery" (Marcus has enough of his own), and all Marcus needs are personnel and engineers that can follow orders, and making a menacing starship does not really require an evil genius. So of what actual use is Khan to Marcus, and how is it that Khan is able to provide such technical services?

Answer 1

The explanation given in the movie, which may or may not satisfy you, is basically this:

Modern day Starfleet engineers are simply not capable of thinking about war the way Khan is.

It's not Khan's technical prowess that Marcus wants, although his "superior intellect" means that he would likely come up to speed very quickly with the rest of the engineering team.

What Marcus wants is someone who thinks like a brutal, savage war-monger. Even though, as you say, Marcus has plenty of savagery in his own personality, he -- and everyone around him -- was born and raised in a time when war was a distant memory on Earth. Now, they are being thrust back into war within a single generation. Remember, the event that "changed timelines" and put Starfleet onto a more aggressive path than TOS happened at the same time James T. Kirk was born.

So, Marcus and his team of engineers certainly have the technical prowess to produce any kind of weapon they can imagine. The problem is they can't imagine them. He wants Khan to look at what the current state of technology is and conceive of things that are three or four iterations beyond what they have now, but are still technically possible, so that Starfleet will be able to gain the advantage over the others races it finds itself at war with.

Answer 2

From Star Trek Into Darkness:

Kirk: Why is there a man in that torpedo?

Khan: There are men and women in all those torpedoes, Captain. I put them there.

Kirk: Who the hell are you?

Khan: A remnant of a time long past. Genetically engineered to be superior so as to lead others to peace in a world at war. But we were condemned as criminals, forced into exile. For centuries we slept, hoping when we awoke things would be different. But as a result of the destruction of Vulcan, your Starfleet began to aggressively search distant quadrants of space. My ship was found adrift. I alone was revived.

Kirk: I looked up John Harrison. Until a year ago, he didn't exist.

Khan: John Harrison was a fiction created the moment I was awoken by your Admiral Marcus to help him advance his cause. A smokescreen to conceal my true identity. My name is Khan.

Kirk: Why would a Starfleet Admiral ask a three hundred year old frozen man for help?

(here, Kirk asked the same question which you asked)

Khan: Because I am better.

Kirk: At what?

Khan: Everything. Alexander Marcus needed to respond to an uncivilised threat in a civilised time, and for that he needed a warrior's mind, my mind, to design weapons and warships.

Spock: You are suggesting the Admiral violated every regulation he vowed to uphold simply because he wanted to exploit your intellect.

Khan: He wanted to exploit my savagery. Intellect alone is useless in a fight, Mister Spock. You, you can't even break a rule. How would you be expected to break bone? Marcus used me to design weapons, to help him realise his vision of a militarised Starfleet.

Khan had a warrior's mind with perfect creativity to build weapons and warships. And, superior intellect also means faster learning ability. Yes, Khan could be replaced with other Starfleet officers, if not one, then more than one (for example, Spock could also be great asset in the project as pointed by Khan himself). But, the big problem was that Admiral Marcus was going to violate lots of Starfleet regulations. He even erased the torpedoes from Starfleet records, for example.

Carol: Are the torpedoes in the weapons bay?

Kirk: Loaded and ready to fire. What are they?

Carol: I don't know. That's why I forged my transfer onto your ship to find out why. I do apologise for that. By the way, if I caused you any problems, I am sorry. I'm Carol Marcus.

Kirk: James Kirk.

Carol: Torpedoes. My father gave me access to every program he oversaw, then I heard he was developing these prototype torpedoes. When I went to confront him about it, he wouldn't even see me. That's when I discovered the torpedoes had disappeared from all official records.

Kirk: And then he gave them to me.

Carol: You're much cleverer than your reputation suggests, Captain Kirk.

Kirk: I have a reputation?

When Admiral Marcus found Khan, he hit the jackpot because he could use his frozen crew to blackmail him and make him do his bidding in any way possible. Not easily possible with other intellectual Starfleet officers because they weren't invisible from the world.