This is straying into the trad jazz thread of earlier. Foolestroupe - you seem to be suggesting that the instrumental fills came in after the song. whereas in trad jazz, the fills were fundamental, to allow musicians to improvise their version of the melody, or the chord sequence.
More cross-over than I had realised (not that I given it much thought)
There must also be a commercial element. The need to get a song up to the "desired" 3 mins duration.

Alex