Lets throw a wrench in the whole wall block concept. What if we could rotate slabs? Put a slab up on edge and we have a wall block, lay it down we have a slab. Being able to rotate slabs from flat to edge could kill 2 birds w/ one slab 
I don’t see a problem with it being impassable to AI’s, as I understand it even if NPCs are added they wont be a particularly large factor in the game.
The only possible exploit that I would see that would affect game play would be to bait a monster and then run through a gap so they couldn’t follow you, but if the monster can’t make a path to find you I think it would just walk away.