I think they do enhance the experience - but for practical reasons they should be limited to high impact scenes (as suggested earlier). It is a simple resource argument: VO + modelling/texturing + mocap etc are very costly in terms of person hours and thus money. I would far rather that limited pot be spent on improving gameplay or expanding locations. If they were mandatory in RPGs, than any RPG that doesn't have them would flop - that is clearly not the case.
