I liked how it was done in Baldur's Gate (and all of the other Infinity Engine D&D titles). No respawns, but resting in an area or fast traveling had a chance to trigger an ambush. You could use these to grind if you chose, but otherwise they served merely as a deterrent to excessive use of rest breaks (without save scumming, anyway).
D:OS doesn't really have a need for "rest breaks," however, so the main reason for such a system's existing is notably absent here...
not just BG; many old school CRPGs don't have farming xp as game feature. basically, when you clear one area, it's done.
the fast travel encounters in Fallout sometimes are even deadly~~"
even now, Bioware pretty much stick to the no respawn tradition (though I think DA games still get too much grinding)
it's not about single-player or mmo game types; respawning enemy is a norm in Action RPGs (Diablo type) and JRPG, that's about it.