I would guess that the girls move around for the same reason workers at any other profession do, Money, P.O.'ed at management, better work conditions/treatment, and to stay busy.
As for STD's: It's still a crap shoot. It only takes once to catch something, but I think you are right about it being relatively safer. Lower #’s of client per day/week, and I would like to think better class of client as well.
Not fact, just my $0.02.
LP