Well, I could think of a couple of reasons:
-Have you ever been in a 747 in Bankok with 400 pax on board and 40 degrees C. outside with no APU available?
-On a 2 engined plane the APU is also the backup if one generator fails. If you`re headed for an area with bad weather, you might wanna have it fixed.
Just 2 not too far fetched scenario`s