Well, I like both RPGs and FPSs. However, I play RPGs for the story, involvement, etc., while I play FPSs for the fast-paced action. IMO an FPS is best played against other players, so the SP campaign is more or less just to practice. I played HL's SP campaign, but I didn't actually find the story that great. It didn't even take a major part in the game (mostly explained only why you were where you are, and who you need to kill next, and with what). I never viewed FPSs as in need of storyline (to be honest, I don't want to be bothered by it, and find it almost a bit silly if they try to have an in-depth story).
I have, however, also considered more FPS-like combat in PS, especially for ranged weapons and magic. However, while it would certainly be nice to be able to actually snipe, the major problem is that with relying on player skill, you more or less exclude a large portion of possible players. It would be rather sad if we were to lose good RPers just because they can't, for whatever reasons, handle the controls well enough to kill even a rat (even if we assume an equally perfect connection for everyone). We already see this in the so-called "tactics" that used to be employed in duelling. Thus I think the combat of PS should be more or less fully character-based and automated, and overall play a way smaller role than it does ATM (also, in an RPG, I don't like to be bothered by combat, regardless of it's form; it's not even close to realistic, anyway).
Therefore, I don't see much benefit in a mixture of FPS and RPG.