Here is my two cents worth. As a cinematographer/photographer, (and skier) I use polarizing filters in front of my camera lenses quite often. I also wear polarized sun glasses (glass, not plastic) and swear by them. Polarized lenses (glass or plastic) work in the same way that polarizing lense filters do. They increase contrast, increase color saturation, and help to remove reflection or glare. They are totally dependant on the angle of light and orientation of the polarized lense itself. In other words, if you wear a polarized lens and tilt your head from side to side, you will see reflections appear and disappear. This is the orientation of the lense itself. If the light source is behind you at about 45 degrees, and the lense is orientated to fully polarize, the results are quite stunning. If you turn 180 degrees around and tilt your head, or re-orientate the lense, the effect disappears, and the polarized lens has almost not effect. Thus, in flat light conditions, there is very little reflections to deal with, and the light sourse is extremely non directional, rendering a polarized lens almost useless, other than its ability to increase contrast. But even a plain grey lens will increase contrast. Polarized lenses are great on sunny days, and when you are in just the right position, they are incredible to look through.
: I find them a must for driving as they remove a lot of windshield glare.
I would highly recommend quality polarized lenses (which do differ in quality by very large degrees) for sunny ski days. Just keep a good pair of yellow or rose goggles around for flat light. Unfortunately, if its foggy enough, even those don't help.