I wouldn't buy it either. It's not quite as practical as you'd imagine I think. Having one screen on the console means you have to have the console in a position where you can see the screen. Unless you're the type who normally plays with his Playstation at his feet, most people won't find the space or spend the time moving the console to that position whenever they want to play.

You mentioned the GBA-GC thing. The DS will be doing that with the Wii as well (first game is Pokemon), and the PSP can interact with the PS3 as well. I remember seeing some racing game where you use the PSP like a rear-view mirror. Portables-as-an-extra-screen work since you can move them wherever you want as opposed to having to move a larger console with a screen around to an optimal position.

A game would need to have a good hook for there to be an extra external screen. Just using it as a menu selector is like using the 2nd DS screen as just a map. Convenient, but not neccessary.