I don't remember if the goblins fled from him in the book. Are we going strictly by the Peter Jackson films? A lot of stuff that wasn't in the films is being added to RJ RotWK.
Also, he could be added to Mordor instead of the goblin faction (which is where I'd rather see him anyway). Though goblins could keep him as a summonable, but that goes back to your argument. If we're going by the PJ films, why would goblins summon a creature they're afraid of? Wouldn't they run away as soon as he was summoned? There's really no difference there between a permanent and summonable balrog, then, as goblins should be afraid of him and not use him either way.
Perhaps the current Uruk Tent could be replaced with something that resembles a run-down Gondor barracks, cover it with banners of the red eye, or the emblem of Minas Morgul, give it an FX to make glow with Morgul sorcery, and call it a Morgul barracks.