Two reasons.
1) Cole Hamels spot will be due up, expect Matt Stairs to pinch hit. From there Jimmy Rollins and Jayson Werth are due up. Allowing Balfour to exit before the lefties comup because...
2) In the 7th the Rays will send 6-7-8 to the plate, if one reaches the Rays could pinch-hit for Balfour, if not the Rays could double-switch and take Rocco Baldelli out. Then bring in Price/Howell/whomever while getting an extra inning from a rested Balfour.