Seven's "catsuit" was initially meant to be a garment that protected her skin as it healed and to replace some of the functions of her Borg exoskeleton. (According to behind-the-scenes interviews, the high heels allegedly contained extra bits of tech that she couldn't do without, like balance-enhancing mechanisms. Supposedly the Doctor placed them there to avoid unsightly bulges elsewhere.) Although that doesn't really justify why she kept wearing it for four years.
As for Troi, well, we know that Betazoids aren't much for body modesty. Maybe close-fitting garments are simply their fashion.