Yes, instant replay in baseball is a good thing, because one bad call, even early in the game, can change the whole game. If a player gets discouraged because of a bad call, they might not play as well in future games. It is important for the integrity of the game for people to know the calls are correct.
The fact that bad calls are rare in baseball does not mean they should not have instant replay. Good calls and bad calls can be subjective, and I think it's only fair that baseball plays be reviewed now that the technology is available. Some bad calls have mattered in very important games.