Max...plastic plants can be harsh on a betta's fins because when they swim through them they sometimes get torn...which opens them up for finrot. They're fins are very delicate...anything in there with sharp edges or ruff surfaces can do damage to them.
Having said that...I have 2 bettas that have plastic plants in their tanks..(bought before I knew better)..and I have never changed them because these two fish NEVER have fin issues. The rest of them however seem to tear thier fins just by LOOKING at them ! I always have silk or live plants and smooth stones on the bottom of my tanks, instead of the average gravel...(cept for those two easy keepers)