IMO this is where the balance comes in. In the NBA, a sub .500 team might get in the playoffs but they are never going to beat the elite teams. In baseball getting into the playoffs gives you a chance to win it all, so expanding it too much would make the regular season almost irrelevant.
But if it's only expanded a bit, it's unlikely we'd see "terrible" teams making it. You'd have a hard time convincing me that if say the Red Sox had made an expanded playoffs last year, gone on a great run and won it all, that they would have been undeserving of a title. In the same way that if the A's (who finished 10 games behind the Astros) had gone on an epic postseason run and won the World Series, we'd be talking about the amazing postseason they had and not the fact that they weren't the best regular season team. I mean the Nats didn't even win their own division and they are the current champs.