Then he will take the AOE damage from Junk/Pharah while trying to fix the turret. And his 50 or what ever heal every hit is lower than most damage you will dish out on the turret.
I honestly don't get why people have such a huge problem with this thing.
You can also just get a tank to soak up the damage while any damage dealer destroys it in seconds, and if Torbjørn is standing there trying to repair it, kill him as well. Fuck, just charge the thing as Reinhardt and laugh at Torbjørn while you smack him down with your hammer.