Well as you wrote it it seemed that no matter how well or poorly you succeeded on a stealth check you'd give the same flat bonus to the stealth check. That just seems as a binary interpretaion in relation to something that is actually dependant on how well he sneaks up on the guy - how close can he get, how much time does he have etc. could easily influence how easily he can kill the guy. Plus although he manages to sneak up on the guy a threath may indicate him kicking a stone the very moment he strikes, so the guy moves a bit (a black die to the melee check).
Adding two boost dice regardless of how he succeeds his stealth checks just seems binary, when the success of the first logically influence the success of the second directly
So a simple success doesn't gain you any Bonus dice but you are still hidden and can delay your attack and attempt another Stealth roll, or not attack and do something else (maybe you notice this isn't a minion but a stronger opponent or whatever), or just go ahead with your attack anyway. The point is a simple success with Stealth gives you exactly what it should and no more = you successfully moved without being seen, you just didn't quite set yourself up for that video game kill.
This is the whole point of the narrative roll system, that you are describing the whole scene not just an action.
Edited by FuriousGreg