As I see it, in a very broad and general way, your avatar is...well...an avatar; a electronic manifestation of the physical you (which I suppose is the opposite of a traditional avatar, which was a physical manifestation of an incorporeal being/principle). As such, I don't change my avatar, as it would be effectively changing what I look like. The only change of avatar I'm likely to be making in the foreseeable future is if Sean ever finishes the Multiworld Madness group shot, in which case I might switch to a cut-out of my character. Like strife said; it was hand-drawn to represent me.
To me, changing an avatar is like getting a new style of hair cut; people won't exactly recognize you at first glance, but then look harder and notice the similar features (namely your name). Given a day or so, and the new look will become easily recognizable. Though, if someone doesn't change their avatar for a very long period of time, and then does something completely different, it might take a few days before I really become accustomed to it. It's like if you had a friend, who's had moderate-length beard as long as you've known him, who suddenly comes over clean-shaven.
Of course, having a spinning avatar that's likely to cause seizures could be compared with someone who dies their hair a very odd shade of lime green. Sure it will attract attention, but no-one wants to stare at it for an extended period of time.
However, for all that, I don't really have a bias towards people's posts due to what their avatar looks like. Unless of course, they're imitating another person's avatar, in which case I see the original person who had it.