If dragons destroy cities you can be sure it will be a scripted event.
I'd agree, the problem of cells could probably be worked around, but I think we can safely assume that if there are only five major cities in the game, Bethesda isn't going to make it possible for any of them to be destroyed permanently except through scripted events. In short, if dragons destroy any cities, it's going to be part of the story. Now, they might start fires in cities, and maybe kill non-essential NPCs, maybe we'll even get some scripted environmental damage like broken walls or something if a city is under attack from a dragon, I'd imagine it wouldn't be impossible for the game to detect if a city is being attacked and activate a script that causes some destruction, but I doubt we'll see entire cities destroyed as part of such attacks. Any damage such attacks might cause other than the deaths of NPCs who won't be replaced will likely be repaired. And I certainly can't see us leveling entire cities, as that requires destructible environments, something I'm pretty sure we won't see. I suppose it's conceivable that a few objects might be possible to destroy, like how in Fallout 3 cars could explode when shot, but I doubt that would happen on any major scale.
Where has it been said that dragons can destroy cities? They can attack and terrorize it, but I don't remember hearing anything about destroying it.
It hasn't, to my knowledge. But it seems that some players automatically assume that "attack" translates into "destroy", whereas its entirely plausible that dragons will do just that, attack cities, but not actually be able to destroy them.