The GBA is short two buttons compared to the SNES, the X and Y buttons. But Nintendo has found ways of getting rid of those button's functions, and it is also quite comfortable
Thats the thing though, having only B and A does not work for a huge amount of games.
eg Any fighter but ESPECIALLY both the streetfighters. You can't have two buttons with a streetfighter game, it's criminal
All the Tony Hawk games
Some of the ports etc.
It was a poor design decision IMO. I can understand why they didn't add it to the SP though because they would be splitting the market into two which is never a good idea (just ask Sega )