For defense or yourself or others that you care about, obviously, but not just physical defense. Defense of emotional wellbeing, feeling, status, etc. can all warrant a good ass-kicking. If someone is threatening you, someone you care about, something you care about, or something you want, ass-kickery may in fact be in order.
I once had to kick a bit of ass to get an annoying, drunk, belligerent freshman to leave me the fuck alone, for example. I was not in physical danger, dispite her drunken "punches," but my sanity and patience were in danger. Thus: ass-kickery.