I change mine regularly to match my mood, or to showcase a new prouduct, or announce a sale, etc...
I figure my avi is like my mood ring. Some days it's professional, some days it's a cat. I get compliments on all of them, I can't say if one works better than another since they all work in their own way.
I know some like to keep it consistent, I get bored with the same look and have to switch it up once in a while. Same with my banner, although my current one has lasted the longest, I will throw in some holiday ones here and there.
I think I do this because I make banners and graphics, so I have the capability of changing mine whenever I feel like it.