That and they're harder to get out of. I feel like if I put my road pedals on my mountain bike and rode with my road shoes, at some point, I'd inevitably end up on my side on the trail, still clipped into my Keos.
Hike-a-bike does happen and the slick road shoes can be a problem. I've had trouble climbing some steep stuff with my mountain bike shoes, I cannot imagine doing the same with slick road shoes.
Not me. Just the first ride that I thought of when I read your post. Steeper and narrower than they look. If I were a better and ballsier rider, I might be able to get down (alas, I am neither). But I have never seen anyone get back up on their bike, though I guess maybe some crazy trials guy could do it if they really wanted.