The thing about Niki is that, yes, she's desperate -- but she's in control, mainly. She owns her sexuality -- she's not being controlled by anyone else, except Linderman on a couple of occasions (and he controls everyone.) And she's not ashamed of herself. The fact that five years later, despite Peter, she's still doing it, says to me that she feels in control when she's stripping -- she's not exploited. Not sure if that's clear, but it's the best explanation I can give.
As for Jessica -- I really liked the way she was developed, because she started out as a 2-D villain and became a character I felt sympathy for; she was broken and on some level knew it, but she wasn't just a broken person -- she would've done anything for Micah just as Niki and D. L. would've, and she was equally dedicated to taking care of Niki. If she's gone next season, as I've heard speculated, it'll be because Niki's finally claimed her power and Jessica feels no need to protect her any more. (And I think Jessica is actually the sister, not just a case of MPD -- Niki has super-strength, so Jessica could well have had the power to transport her mind into other beings.)