No, the rubble is not destructible, it's the same kind of stuff used in the destroyed rooms.
So I'm guessing rubble is a no no in every other type of room?
No, it can be used. I've used it a bit in some of the rooms I've made recently. But, it should be used sparingly, and with the realization that it acts like a wall, and a player can't access it. (In other words, if you aren't careful, it can block off a room.)