Sounds a bit too much of artificial game balance to me.
If you backstab someone, then you have an adventage already. This is no fantasy RPG where the rogues (bad depiction, mostly they are just stupid assassins) need a burst damage to kill some mages.
As for the buckler with dagger combo, i use it with my siege ladder setup, and the first thing i do, after the ladde is deployed, is to look for another weapon.
You can kill someone with it, but it is not that easy and takes some stabs.
But after all it is only a short knife, while the others have big swords. So why you should have an advantage over them with a tiny dagger?