There's plenty of other really good games you haven't tried though, give some of them a go, rent some or something and buy what you like.
For instance there's:
Animal Crossing - allot of people weren't interested in this game until they tried it. Check out the forum at IGN, there's a thread about it there that shows it can suprise you.
Eternal Darkness - you have to give this a go, its not a game that anyone should pass up without giving it a try.
Rogue Leader - Everyone should at least try this game, its a very good game with brilliant visuals, if your a Star Wars fan then its a fantastic game!
Pikmin - can be a very addictive game, again at least try it.
Resident Evil - well I suppose if you hated past Res Evil games then you'll hate this, unless your a graphics whore. But if you do like Res Evil then its deffinately worth buying.
Super Monkey Ball 2 - this is another one of those games were if you don't at least try it you may miss allot of enjoyment.
Super Smash Brothers - a great game IMO, but if its not your sort of thing then not worth buying I suppose.
Wave Race - the best water racing game ever made, again you may miss something if you don't try this game.
I'm sure I missed a few.
There's plenty of games coming in the next couple of months that won't be available on your XBox.
Metroid Prime
Star Wars: The Clone Wars
Res Evil 0
Wario World
Star Wars: Bounty Hunter
Die Hard Vendetta
NASCAR: Dirt to Daytona
Godzilla
Golden Sun
Mario Tennis
Rally Championship
There just some of the ones that interest me of course, you may not be interested in some of it, but I'm sure at least a few of those will interest you.
As for your comment on Nintendo and Rare being the only devs that were worth having a GameCube for.. nonesense IMO. Maybe so far Nintendo and SFA is all you would, at first glance, get a GameCube for (although you haven't tried allot of great games like ED ect AFAIK). But surely you can see there are allot of really good games not made by Nintendo or Rare on GameCube, that allot of people would buy a GameCube for, after all Rare only made one game anyway. I'm sure even you can find quite a few none Nintendo games you want on GameCube, either now or in the future, that will not be on your XBox.