I honestly find myself asking that question once a month, though I doubt there's a Wii title I'd like to buy, or even play, releasing each month. I'd love to play Red Steel 2, mess around with some of the fun multiplayer experiences on the Wii, or ready myself for the next Zelda. But is it really necessary?
I guess there are times I feel like I'm missing out on something, Oops! Prank Party withstanding. There seem to be a handful of great games and experiences on the Wii that are obviously not on the Xbox 360. Being a single console owner certainly had a "grass is always greener" effect.
If I had the money, I'd buy a PS3 straight away. I already know of several game I really want to play on the platform, but the Wii is a bit of an enigma. Sure I'd love to play Super Mario Galaxy 2 or some of the Wii's few core games, but is there enough to justify buying a console I've already give up once?