I'm also in the group that was never really sure what that button did...until this morning.
So I leave for work and it's cold and icy. I figure, "Let me try this thing out and see what it does." Very shortly after I turn it on I smell something not normal. I keep driving, turning things off and on, trying to place what the issue is. Then I notice what appeared to be smoke or steam rising up from underneath the passenger side wiper blade. I think, "Hmm, that ain't good."
Now, my windshield is shot to shit. I commute 40 miles to work through road construction and my windshield looks like a prop from a James Bond flick. I purposefully decided not to replace it until the stretch of construction is finished. Probably in 20 years.
Anyway, I get out to inspect the source of the smoke and I can see a little glowing ember where one of the impacts of road debris hit. In addition to chipping / cracking that spot on my windshield, it also broke or weakened the heating element embedded in the glass. My immediate thought was, "Damn, this is gonna be an expensive windshield replacement."
Fortunately, I don't really need to use this feature so I'll probably still wait for the road construction to be completed before replacing. I just figured I'd pass along in case anyone has chips way down at the bottom of their windshield. I doubt anything serious could result, but you never know.