It'd be about perception. Fans that think the game is currently too long might reactively check out a shorter game. It's less subtle than other attempts, and thus more attention getting. any one inning will last just as long, if not longer. You won't get rid of the 25 man roster, and will still have a lot of relievers. The manager can come out in the 4th/5th and change relievers repeatedly for matchups, slowing everything down. Plus, it would be ridiculously low scoring.