The purpose of a website's icon — or any icon, for that matter — is to provide a visual way to quickly find something in a list. Sometimes, the icon represents some abstract concept; in most applications, the "save" icon is a floppy disk, even though they're nearly obsolete. However, if the icon is unique, experienced users have no trouble connecting it with what it represents. I use icons exclusively for my bookmarks toolbar:
Of course, this only works when the icons don't change. Google has recently changed their icon again, just as I was getting used to the second one. Call me old-fashioned, but I happen to like the original Google icon.
I can understand changing the logo on the front page for special holidays (which seems like just about every day now), but icons shouldn't be changed just for the hell of it.
