I do it, but not indiscriminately and always safer sex stylee.
My current bf does not like any ass play, so I can only reminisce. But, always fun to see a guy respond with a combination of shock, anticipation, and excitement when my licking and sucking doesn't stop at his balls.
The heavy breathing and moaning in response to my licks and teasing are big turn-ons! Is it something I must do? No. But, other than just being fun, I do it for either the "Oooh, you're such a dirty girl" factor or because I want to open him up to new experiences.