The whole selection of All-Stats needs an overhaul to begin with. Why are the managers making all the choices? If this is supposed to be a fan ballot, why aren't fan votes choosing All Stars? I get that World Series home field advantage is on the line, but the current process is just silly.
The fans vote for the starters, the fans vote for the reserves. This is standard in all league all star games if I'm not mistaken (it's certainly the case in Basketball). It's a mix. The fans care about seeing their favorite players, but a good team composition requires more thought than just majority vote. Decisions like how many pitchers to carry, for example, is something that a manager would have to make.
The final player is just a final toss to the fans who missed their favorite player getting in. They won't play until the end of the game no matter what.