There's a plethora of reasons for you to purchase one. It's got some of the best games currently on the market, it costs 100 bucks, it's an extremely durable piece of hardware (I'd estimate that I've played somewhere between 700 and 900 hours on the machine) and it has the bar none best multiplayer titles you can find.
And before anybody starts screaming "Halo" at me, that's one game the XBox has. And regardless of how entertaining that game is (I actually played it for an hour or two a week ago, and it is pretty damned fun), it provides nowhere near the level of multiplayer amusement that Super Smash Bros. Melee or Final Fantasy: Crystal Chronicles can provide.
Robo, I honestly have no idea why you're still "thinking" about picking a Gamecube up. You should already own one and be knee-deep in Mario games by now.