Plastic is actually a porous material so it will not keep your beer as fresh as glass over long periods of aging. This may not be detectable though. The only other possible issue could be plastic residues that could impart a plastic flavor to your beer. Other than that, if you have been using the plastic for a while and not had any problems then I don't see any reason to change to glass. But I personally prefer glass. I have a glass carboy that has been in the family since 1962 and it is in as good of a condition as the day it was made.