I don't think they will produce a good narrative combat module at all. Either:
a) Stunts are better than your normal attacks, so you do them all the time, or
b) Stunts suck and aren't worth using.
Or you limit them to once a fight or something, I don't know.
Stunts have to be weighed against the utility of making basic attacks. So if you cannot ever envision a situation where stunting is superior to hitting your opponent with a half-brick inna sock, your stunt is bad. There's no reason to use something that inflicts status effect X, Y, or Z when you have a perfectly good sock which inflicts the status effect "unconscious or dead" on a hit.
Now, with that in mind, then there are plenty of reasons where you would want to use a stunt even when you're fighting mooks, and they are all situational.
If you suspect that you won't be able to kill your opponent for a good long while, a debuff is the pretty obvious winner, and if you've got a crowd of mooks nearby, something that debuffs them all may very well be more useful than killing off one or two of them.
It's also possible that your immediate priority is neutralizing or hindering an opponent rather than causing them harm. Maybe you're fighting good guys and don't want to hurt them, the elven prince is about to put on the cursed crown, someone's about to strike a gong and wake up the tarrasque, whatever. For whatever reason, swording the obstacle in the face is going to have a less desirable outcome than stunting, so you stunt.
In fiction, stunts also usually come into play when your normal mode of attack fails to produce useful results. A fighter going up against a golem whose iron carapace deflects all his attacks will try tripping it, finding a weak spot in the armor, dropping a stalactite on it, or whatever. Again, this is situational. If he were going up against a flesh golem, it probably would be more efficient to hack it to bits than it would to spend several rounds luring it to the edge of a pit and then slipping through its legs to give it a push.
But it's entirely possible to have a stunt system that players will use, so long as you accept the fact that the most obvious and logical answer to the question, "What do you do to the goblin that popped out of the coffin?" is "I introduce it to my spear until it stops moving." That answer is predicated on the assumption that your normal attacks actually pose a threat to the local wildlife, though, so if choosing Answer B: Spear-stabbin' does not actually lead to a dead or severely wounded opponent, then yeah, stunts are going to be too good or even worse.
