Britney doesn't care enough about what's going on to use gimmicks. She does what she has to and then gets the fawk off the stage. Hell, sometimes she doesn't even do what she has to on stage (TTWE at the BBMAs anyone?).
Beyonce has never.
Lady Gaga doesn't have to, but chooses to unfortunately.
Katy... uh, yeah. She does. Sorry Katy Kats.
I don't think that the whole "sex" thing is a gimmick of sorts. I think Rihanna is just obnoxiously horny all the time and is willing to comply with her label's request to be sexually provocative.