Most of the teams that have already clinched a postseason spot aren't exactly playing good ball right now. Here are the records over the last 10 games (Saturday's night games not included):
Red Sox: 4-6
Angels: 5-5, Last 20: 10-10
Phillies: 4-6, 3 game losing streak
Cardinals: 3-7, 8-12 last 20
Dodgers: 3-7, 5 Game losing streak, Last 20: 10-10
Rockies: 7-3, 5 Game winning streak
If the Tigers win the division, they aren't exactly on a roll either. All the NL division leaders each recently went on a 5-5 stretch.
The only teams that are hot right now are the Yankees and Rockies. It could be argued that because the regulars are not playing, teams aren't winning. However, most of the NL teams have been fighting for home field. The Red Sox were in a bad slump before they clinched.
Also, it is not good to enter the postseason cold. For those being rested, long layoffs are not always good. Remember the Rockies in 2007? Their hot 21 of 22 streak carried into October, however they had a long break between the NLCS and World Series. It backfired and they lost to the Red Sox. It's okay to give a player rest; a day here, a day there. It's a long, tiring season. But sometimes too long can be trouble. If a player is slumping it may help, but if they're playing good ball, then it may turn into a problem, taking that player out of their rhythm.
Most of these teams are hopping into the postseason on one foot.