It's not so much the excitement of the division races as it's increasing the amount of teams whose fans are still interested late in the season. If you just took the 4 best records, you're probably going to lose out on a lot of tickets sales and ratings from teams that are out of that exclusive race. People might complain about the integrity of the playoffs, but baseball is still the most exclusive of the team sports as far as qualifying for the postseason. I don't understand how choosing the 4 teams with divisions keeps more teams in the race than 4 teams with the best records.