You run the risk of image degradation. A risk that I, personally, won't take. Your mileage may vary.

Hello all!
Im using my UV as a protector for my lens and I am not fond of unscerewing it every time I want to mount a PL filter. Is it THAT bad for picture quality to stack the PL on the UV (assuming no vignetting is visible)?
Thanks
Chen

It depends on the quality of the filters involved, but in general, yes. Don't put any filter on your lens unless there's an urgent need to do so.