I say half the games didn't really use L2 and R2 for anything useful. For the part that does, most can live with them being mapped to something else in the keyboard.
Obvious exception to that is the Megaman games of course. It *will* be kind of awkward to play Legends without R2.
But anyway for just a small fraction of a fraction (PS1 emulation) of uses, it's not worth the trouble ( = lots more thickness) to add 2 extra shoulder buttons. Heck, Sony didn't put L2 and R2 in THEIR portable.