Nets would be cool for many purposes besides. I think this could work better once knocking someone unconscious without killing them works (although there's always good old pain isn't there)
Capturing without cage traps is a very good idea.
I think Toady is actually working on non-lethal combat options for subduing enemies in this upcoming release although I don't think he's implementing it quite in this manner. Adding things like nets, bolas, etc. to the game and using them as a means to capture enemies, particularly those that have already been weakened or knocked unconscious sounds like an excellent addition to the game.
It would make a good alternative to the ridiculously broken cage traps which are currently our only means to capture enemies. Toady really needs to get around to MASSIVELY nerfing and reworking cage traps but we will need some sort of alternative means to capture those stronger foes that should logically be immune to them.
As it stands right now, a rickety cage made of sticks by a neophyte mechanic is a God slaying super weapon capable of instantly defeating mega beasts that have slain whole armies. If you throw a little web on your cruddy stick trap, it can instantly take out an eldrich abomination made of living magma that is worshiped as a dark God by the civilizations it has been terrorizing since the dawn of time. It is perhaps the single greatest let down in dwarf fortress to see such foes be defeated in such a laughably cheesy manner.