The only purpose for overturning the call would have been to give Galarraga his place in history (which he'll have anyway now) and give him the recognition he deserved. From that perspective, I would have supported it.
But overall, it sets a dangerous precedent, because as has been said, you'd have to give the same consideration the other way (ie: if a bad call awards a perfect game or ends a game prematurely, you'd have to reverse it).
As well, history in every arena (not just sports) shows that you give people an inch, they'll try to take a mile. It would have started a snowball effect that surely would have led to reversable calls in really bad places.