Well, back when Civ3 first came out, Expansionist and Commercial were considered two of the weakest traits. So a Civ that combined those two just *had* to suck. And when it's UU is a Naval Unit, to boot?
Of course, now people have played, and experimented, with all the different civs, and the conclusion: the traits are pretty well-balanced, after all!
Expansionist can be great for the early game, especially on Pangea maps. Safe popping of goody huts, lots of free techs, etc., can give you a solid early-game lead.
Commercial doesn't seem as strong early, but by the mid to late game, the added gold it generates is a powerful boost! More gold for buying techs/Luxes/Resources. More gold for upgrades. More gold for Science/Lux sliders.
Of course, the Man-O-War is still a pretty poor UU.
For myself, I usually play as the Americans. I love the Industrious trait, and combined with Expansionist, I can build up a solid game pretty early. (Yeah, the F-15 is not exactly the best UU either.

) But I do try to play the other civs, at least occasionally, just to understand how everything works.