If the 'Cube had all the decent PS2 exclusives like Devil May Cry, Final Fantasy X, Grand Theft Auto, Kingdom Hearts, SSX, etc then I'd probably only own that console. Combined with all the Nintendo-produced games, it'd easily be better than both other consoles.
Unfortunately, third-party developers feel the need to either a) boycott the GC altogether, or b) whip up a shoddy conversion straight from the PS2 game just to make some extra cash on the side (SSX Tricky anyone?). Systems like the X-Box are going to ruin the industry by adding their online/dvd/stereo/sandwich maker capabilities to consoles. This leads to X-Box fanboys bleating on saying "omgomg xbox is so powerful u can cook ur dinner in it lol."
I buy a system to play games, not to fly it to Mars.