I think it's not that gowns aren't useful (so far as fancy clothing is "useful") but that the same suit is good for different events than the same dress is good for. If that didn't make sense, I mean that there are more levels of formality for girls than for guys, so a dude can get away with only having one suit. But for all the occasions he wears that suit, his lady friend might find that she needs a sun dress, a little black dress, an informal but winter-appropriate dress, a true gown, and business wear.
Personally, I have a sun dress, two formal and one informal black dresses, but I only wear one of them, and in some closet at my parents' are an old prom gown and my wedding dress.