The way I play, characters have a ery limited amount of HP (base HP = Con score, three HP progressions. Good, gain 1 HP every level. Bad, gain HP like the Cleric's base attack. Poor, gain HP like the Wizard's base attack).
With this system, every hit counts, so I give all characters a Base Defense bonus, and have armor provide DR *instead of* AC.
They *really* need that DR when they get shot with blaster rifles. without it, they'd be dropping like flies.
-Jeph