Been away from the MD-80 for awhile, but what I recall from this situation is the concern over something happening to that single engine-driven generator until the APU started. With one generator, the APU had an electric starter assist. Without, you were down to a windmilling start (to preserve the battery) which could take a while if you were not well inside the starting envelope. Not a huge problem in VMC conditions, but IMC was a different story.