Perhaps not for big new Sony titles, but if you're a family / have kids you definitely want at least two as there's still lots of good couch multiplayer games out there.

If it's just for you then possibly for stick drift (rather than getting them later when they've gone up in price again) or because they still use a built-in battery instead of rechargeables, and they don't last very long by comparison (and that play time gets even lower as they age obviously).