I've always been really bothered by in-character reasons for the costumes. I can just about deal with Huntress' outfit because she's not showing off for a bloke, it's all about her vanity and her (justified!) pride in her training regime.
In the same way, Zatanna's fishnet's and fetish-formalwear look is simply because she loves "playing dress-up". Again, it's simply because she thinks it's a fun look.
Power Girl on the other hand, makes me flinch. As I recall it, and I'm quite happy to concede that I may be having a moment here, she doesn't have an S because "oh, Kal, I'm not good enough to wear the S compared to you, so I'm waggling my tits at the world until I come up with my own symbol."
...and I've just realised the scans are gone that could back up any of this. Arse.