I just bought a new frame and fork because I wanted one, not because I needed one. At all.
I often have more fun on my cheap steel singlespeed than on my full carbon superbike. This is because I'm less worried about damaging the steel bike and just ride the crap out of it on/off trails, on group rides, dirt roads etc. Also, I'm not that convinced it's all that much slower than my superbike, especially in groups.
I'm buying a set of carbon clichers even though alloy clichers are almost certainly more practical, just as light and almost certainly safer. This is probably a dumb decision.
Most shamefully: I didn't want to buy my kids a piano this year because I didn't want any hassles from the wife about how spending money on more uneeded bike crap isn't in the budget. I'll end up getting the piano, but I'm not proud of my initial reaction.
On second though, perhaps equally shamefully: I enjoy the movie "Blue Crush" and not just because of the hot girls.