I use it for 2 reasons
first because it annoys me that women are supposed to be defined by their marital status in a way that men aren't (unreconstructed old feminist, me)
second because when I married DH I kept my own surname so it feels odd being Miss (because I'm not) but equally odd being Mrs Visiter because that's my mum!
third because I like pissing off call centre people when they say "is that Miss, or Mrs?" in a perky voice, and I say "MS" in a passive agressive growl
Seriously though I don't really care if someone calls me Miss, Mrs or Ms, it's just the box I choose to tick on the form.