Polarized is bad for two reasons. Polarization is directional...
1) Airliner cockpit windows are polarized in a manner designed to not interfere with your ability to see other aircraft. Unfortunately this causes visible interference with when you wear polarized shades (it's not really that bad, but it's noticeable).
2) This is the biggy...the light reflected off of another aircraft may be inadervently polarized by the reflecting surface. If this polarization lines up right with your sunglasses, it will turn the other airplane invisible (until it changes angle or gets really big
).