Karune:
While I don't know about the 'main idea' of the add-ons being detachable is so the other side could steal them, this mechanic is still in the game. The AI does not prioritize the buildings over the the add-ons in StarCraft II. 'Stealing' is also a bit trickier to do since Terran players can salvage buildings like add-ons and bunkers to get 100% of the cost back over X period of time (in which X is subject to balance still of course).