There *ARE* actually games which requires a Dual Shock 2 in order to run.FYI, that was fallback behavior in the original. If you played with a Dualshock 1 (no pressure sensitive buttons), it would register each press at max pressure. I'm pretty sure this was mandated by Sony, I remember similar behavior in Dynasty Warriors 3 and GT3.
Silent Hill 2 won't even start without one and I'd imagine MGS2 would fail to run as well (it's impossible to play on PS2 without those buttons).