One of the true values in the Nintendo universe is the immediacy of the experience. The Gamecube boots in a few seconds, and requires little effort to get a game going. To go play a round of Tribes2 I have to boot my PC (over a minute, and quite likely to complain about something) and then dial in, then start the game (and if that game is Unreal Tournament, that load time feels like days), then connect to the master server, select a game server, then wait for the map load, and then play. sprinkle in random modem drops, or game server drops, and the value gotten from the process drops like a stone.
Back to the Gamecube. It boots quick, plays games well and has good software titles that can be found no where else. For a vidiot game junkie like myself, that's a compelling feature set. Nevermind the Gameboy Advance/Gamecube integration.
Also, I'll be buying the new Gameboy Advance SP as soon as it comes out. Damn you Nintendo.