It's needed for all places where something can be called wrong except balls and strikes. That's more of an opinion of the ump. Fair or foul homers and calls at the plate on the bases are either right or wrong and there needs to be a second look at the call. The argument against it is it ruins the purity of the game. ********, if Doubleday knew there would be a time when people can instantly review the call of an ump, he would have that in the rulebook, if he has any bit of common sense. Also it would legnthen the time of the game, real baseball fans don't care if it takes another 5-10 minutes if the call is right.
I'm not satisfied with just hearing it's part of the game. It doesn't need to be.