Pleating to the stripe was also done for economic reasons:

1) Even in the higher yardage styles preferred by the military it uses slightly less fabric. If the kilt is being made from a tartan where a line repeats itself more than once even less fabric will be used. Black Watch is a good example. It's typically pleated to the stripe using only half of the sett.

2) Future alterations: Pleating to the stripe means there is no centre stripe in the back. Buckles can easily be moved without worrying about the kilt looking off-centre from the rear.