[rlpb]

SIM cards are exactly this, with the added benefit of (almost) two factor authentication. If I have my SIM card, I can be reasonably confident that you can't pretend to be me. If phones managed keys, I'd be more worried that you've managed to copy my key somehow.

[kamjam]

Especially give the sheer number of apps out there and reports of apps doing more than they should, as well as users blindly installing apps with higher permissions than they actually require. This would get exploited very quickly no doubt.