It was Ron Moore's idea to put Troi in the uniform, which Marina Sirtis was eager to oblige with.
Moore himself has commented how ridiculous Seven's catsuit was, calling it "eye candy with no content".
LOL, did he say it with a straight face?
No Content? Is he blind, Seven's catsuit was busting out with contents
Seriously, though, other than Marina Sirtis having wider hips, there was really no difference in the revealing-ness of their costumes. There is no more justification for Troi's outfit than there was for Seven's. They both showed off every bit as much of the wearers body as the other, and neither could be justified with anything other than being eye candy
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?