What did Harry ever do though on most episodes that some non-speaking extra couldn't have done? He was a glorified set-piece. How many Harry-centric episodes can you even name?
Harry wakes up back on Earth.
Future Harry saves the ship from crashing on Hoth...erm, Ice Planet Zero...
The space derby was equally about Tom and B'lanna so I don't count it. The one where the ship's duplicated and one Harry dies I don't count because it still focused more on Janeway. The Chute was equal parts Harry and Tom. Right now I honestly can't even think of other episodes that featured Harry.
Harry was going to be killed off to make room for Seven of Nine until People Magazine put him on a 50 or 100 sexiest men feature and the producers decided he couldn't be fired in light of that so Kes was booted instead. What would have been lost if Harry had been booted instead of Kes though?
At least some of the Maquis were former Starfleet and were given battlefield recommissions by Janeway. Seven wasn't, and had no technical rank so why give her a uniform? Also, a baggy jumpsuit wouldn't show off the only reason she was hired in the first place
I'll grant that Seven was more interesting than everyone else on the show besides Doc, but that could also just be a result of the writers not trying to make other characters interesting. Braga was also dating Jeri Ryan at the time, but I'm sure that has nothing to do with her having better writing and more screen time than most of the rest of the cast.