A plastic wad would give you a tighter pattern, but the problem is, you ought to have some type of lube on a cushion wad when shooting BP to help keep the fouling soft. Does about the same thing as lubing a patched round ball.
If you don't, the fouling will bake on hard in the bore, making loading difficult and act as an abrasive on the plastic everytime you shoot.
Makes it hard to clean as well.

Hope this helps.