Yes I want it. To some extent. Destructable trees would be really nice.
But... everything's that is destructable should very slowly "restore" itself. It would be sad if something got completely ruined. Far Cry 2 had this "restore" thing, but it was too fast. A slow transition would be nice
If dragons attack it would be really weird if everything is 100% intact. Seeing buildings be razed is also really cool. Makes the game a lot more exciting and interesting and believable.
Also, as far as I know, the only time destructable environments "hurt" the performance, is when it's overused at the same time. One building being razed wouldn't really affect the performance at all. Twenty buildings at the same time, and you got quite a big performance hit. Doubt that would happen though.
The only reason I see not to have fully destructable environments would be if it takes way too much time for Bethesda, which I doubt