I for myself use the "fun"-skins for much shorter times than the "useful" skins.
The fun-skins usually reflect my current mood as well as the season and/or holidays. I keep a heap of them around and I switch dem often. For me they are a pleasure to look at whenever I boot up my computer and they often lift my spirit.
As soon as I have a lot of computer-work to do, espceially things like Excel-calculations and Databanking, I find myself going back to the sleek, and stylisch/functional skins (never ever to Windows classic though

).
But then there is the next holiday or a change in my mood....