I think the point that this isn't restricted to sex workers is a valid one.
Case in point, a girl I met in university who was getting married would constantly complain about her boyfriend, act all pouty, and essentially do so to get the men around her to carry her academically. Now, eventually, it would turn into a sexual relationship, so I guess she wasn't just a tease -- but there it was. Once I realized what kind of person she was and what she was doing, I disengaged, but she just moved down the line - getting more blatant, essentially playing the "poor me, my life's too stressful I can't do the work because of it" card and getting other people do either do the work for her, or in at least one case, get an older paper to hand in as her own. Basically, though, it was never a blatant "I'll trade you sex for...", it was always the manipulation, the make you feel sorry for her, the make you feel like you're rescuing her and that it's a real relationship based on real feelings, rather than just a way for her to get what she wanted.
Of course, the lady in question later became a stripper, so maybe my first comment isn't so accurate. I don't know what name she used while peeling, so maybe she's one of the ones that you guys are talking about.