I use grenades liberally when fighting zombies. Couple of weak ones or single strong ones, I’ll melee them. Couple strong ones/shockers, and I’ll gun them down. Mass of zombies, and I’ll circle 'em until they’re moving in a nice tight pack, then start chucking grenades at 'em. Thin the herd until I feel its light enough to clean up through Fire or Sword.
Seriously, a tin can, gasoline, string, and a pilot light is all very cheap, and makes mincemeat of massed zeds.
And NPC’s would just get in the way of that.