Although the usage of a slot-2 device would erase the Guitar Hero Guitar Grip, developers can still gather it's source code and create a new slot-2 device that has a card slot and has four buttons, like the Guitar Hero Guitar Grip.
I think it would be a good idea because:
1. It would allow for more button usage
2. It would be possible to have a homebrew Guitar Hero game for NDS/L
and 3. It would be as big as the original, allowing for storage space, extra RAM, etc.