realskiers

I found that my knees (and ankles to a degree) felt a lot of stress when putting any ski over 100mm wide on edge IF I was tipping incorrectly.  If the way you tip your skis on edge involves throwing both your knees from side to side then you're probably going to feel some serious joint stress in your legs.  If you tip the PMTS way, where the free foot has little to no weight on the ski and the stance ski follows into a tipping angle governed by the tipping of the free ski, you should find greatly reduced stress to possibly no stress at all.  When you tip the PMTS way, you maintain correct skeletal stacking through your stance leg.  This skeletal stacking does not have the tons of knee angulation you'll see in most advanced non-PMTS skiers' skiing.   I learned last season that I thought I was tipping correctly, but clearly was not once I hit PMTS camp.
 
Wide skis actually provide a ton of bio feedback on your tipping skills - if you're not doing it right you're going to be feeling the stress immediately.

High Angles:
I agree that there is a ton of feedback from the extra resistance of wide skis being tipped, and maybe even some good muscle development, but I also agree with Max and JBotti about effect on my technique. When I go back to my narrow waisted skis I can feel (and Coach Lynn can see) that my actual technique has slipped.