The purist I am, I've always been pretty opposed to the idea of instant replay in baseball. Now, GMs are trying to make it happen. And honestly, I'm actually in favor of it.
The proposal would allow instant replay only to help determine home runs calls: whether they are fair or foul; whether or not it actually cleared the fence/line; and whether or not a fan interfered with the ball.
These are calls that could easily make a huge difference in a game. Obviously the biggest memory that comes to mind is Jeffrey Maier reaching over the fence to give a home run to the Yankees in the playoffs against the Orioles.
I don't think that it hurts to take the human element away in these situations and leave it up to instant replay to decide. It's always a controversial decision to make, and at least one of the managers has to come out of the dugout to argue his side with the umpires. It would be a good thing to make sure they get the call right, with little to no possibility of controversy, especially in a big playoff game.
The only thing I worry about with this is the foot-in-the-door aspect. If Bud Selig, who is strongly against instant replay, relents for this change, it could then make it easier for GMs to push for more replay use, and then it could get into the area where I would also be strongly against it. Things like fair or foul down the line calls, safe or out calls, etc. Baseball purists everywhere would cringe.