Voyager: the ship where not everyone is a happy camper
To get at the reasons for having Tom Paris as the USS Voyager's Chief Conn Officer, we must first discuss the objectives for Voyager as a television series.
Both DS9 and Voyager made efforts to depart from some of the philosophies of The Next Generation. For instance, DS9 increasingly attacked the notion of the United Federation of Planets as a Utopia. On the other hand, Voyager attacked (or at least was intended to attack) the notion of an orderly Starfleet starship in which the senior officers rarely make grievous errors and are generally infallible.
Stranding Voyager in the Delta Quadrant and forcing its crew to work alongside a group of very anti-Starfleet Maquis members was a catalyst for creating tension and disorder on Voyager. (Ultimately, the producers creative staff failed to capitalize on this tension by dissolving it too quickly, but that's another story.)
A good example that contrasts the general infallibility of crew in TNG is Harry Kim. Here, we have a wet-behind-the-ears ensign who is thrust by circumstance into a permanent bridge position, putting him at the conference table with the senior officers. And of course, he makes several notable mistakes along the way due to his inexperience.
Now we come to Tom Paris. The producers seemed to want to complete this tension by having a character in need of redemption — someone ill-at-ease with both the Starfleet personnel and with the Maquis who would have to earn their combined trust. This of course was filled perfectly by Tom Paris.
The template for this character was Nick Locarno, a brash Starfleet cadet whose plan to cover up a flight disaster in TNG's "The First Duty" almost cost Wesley Crusher his Starfleet career. Locarno was also played by Robert Duncan McNeill.
TNG itself had already borrowed a "broken" character from this very episode for one of its later episodes, "Lower Decks". There, Ensign Sito (one of the conspirators with Locarno and Crusher) must prove herself to Picard.
The initial plan was to use Locarno outright, but this would have meant paying royalties to the writers of "The First Duty". Also, Locarno was seen as a bad guy masquerading as a good guy, and the feeling was that the character they wanted should be the opposite:
As Robert Duncan McNeill explained, "Locarno seemed like a nice guy, but deep down he was a bad guy. Tom Paris is an opposite premise in a way. Deep down he's a good guy. He's just made some mistakes."
From this realization, the Tom Paris we have come to know was born. Now for the tricky part: did being a pilot come into Voyager through the Nick Locarno inspiration, or was the Chief Conn Officer position an inevitability of the main plot lines of Voyager? Unfortunately, there are no statements from the creative staff to answer this.
My speculation is that it was a bit of both. I believe that the pilot aspect played naturally into Voyager's predicament: neither did the storyline need to be adjusted to accomodate Tom Paris being an exceptional pilot, nor was there a need to update the "Locarno profile" to give Paris a different expertise. Here's why:
Voyager was operating under unusual circumstances with a relatively small crew
The crew of the USS Voyager, Maquis included, was about 145. Kirk's Enterprises had crew complements of roughly 2 to 3 times this many (depending on which Enterprise and which decade of operation), and Picard's Enterprise had almost 7 times this number of personnel. Naturally, those ships would have more trained pilots than Voyager does.
For purposes of comparison and contrast, let's focus on the Enterprise-D as a ship that was nearly contemporaneous to Voyager. Being the flagship, the Enterprise-D was not on a long-term deep-space mission. It was freqently visiting various Starfleet starbases, and during TNG we see it visit Earth a number of times.
New junior crew members, often fresh out of the academy and requiring experience, were acquired by the Enterpise-D at various points. Riker and Troi were in charge of the bridge duty roster, and the "night shift" was frequently reserved for officers, including helmsmen, with less experience. Sometimes, however, an emergency strikes while a junior officer is at the helm, and sometimes another officer is seen to take over. All in all, there are plenty of reasons why we see frequent changes at the helm on the Enterprise.
Now let's turn to Voyager:
- Voyager is stranded in the Delta Quadrant. The crew it has is the only crew it is going to have for the foreseeable future.
- Of the only 120 or so surviving Starfleet crew members, only a handful would be competent enough to fly a starship through unpredictable circumstances. (In normal circumstances, the ship can fly itself if necessary, and the computer can also implement pre-programmed evasive maneuvers in an emergency, but a competent helmsman is required for most real battle scenarios. That's one of the reasons why starships still have helmsmen.)
- Voyager's primary helmsman was Lt. Stadi. However, she was killed when Voyager was flung from the Badlands to the Delta Quadrant in the first episode.
- Without being ordered to do so, Paris grabbed the helm and steered Voyager to safety. He also distinguished himself by saving Chakotay's life towards the end of the first episode.
Given Paris' expertise, the shortage of personnel, and his heroic actions, it was both prudent and justified for Janeway to make Paris into the Chief Conn Officer.
The producers essentially borrowed a "broken" character from a prior TNG episode in order to paste in a certain trope (the troubled but good soul in need of redemption), and this character happened to be a pilot. On the other hand, having a Chief Conn Officer was also manifestly natural for Voyager given its situation, and so one can say that there was a need for such a role, too. It so happens that Tom Paris fills both these roles: he needed saving, and his skill was put to use in saving Voyager.