Along the way you'll unlock loads of characters, so when you take on Bowser, you'll have big hitters like Donkey Kong, nippy fielders like Diddy and classy pitchers like Peach. Instead, of earning anything worthwhile, all your unlocked players are removed from your team and you just have to play in the same stadiums again, only this time it's even harder. Sure, you'll unlock an extra minigame every time you beat Bowser, but these are mostly crap Mario Party-style games that have nothing to do with baseball.
Still, sports games are best played with friends, and seeing as the baseball is very good, multiplayer is great fun. As with all Mario sports games, the genius lies in the little details - the way it uses the Mushroom Kingdom's idiosyncratic scenery and marries it with proper, almost sim-like sports play to create a game that's silly, but never stupid.