Its totally a necessity. Not all actresses or even woman are 'sexy', you can be beautiful or hot or cute but being sexy i think is an offshoot of your personality.
Thats probably why a lot of these actresses just come off as cheap or over-done because their directors stick them in a skimpy outfit and write a scene/song in which they can just stand around and gyrate their hips. -_-''.
I doubt they go out of their way to don the sexy persona.