The uniform rule has some merit. Obviously it is important in relay races. Infact, this past year the IAAF now requires the same color(s) on the back as the front. But it is also important for team scoring. Often there are great dog fights for those last few points in a meet and good coaches have their athletes focus on the athletes that need to be beat to win a meet. Of course, this is even more important in a x-country meet.
In addition there is a safety issue. An athlete that competes in a uniform that could restrict movement or cause an accident (too big, wrong shoes, etc.) may quickly become a legal issue. with the coach and school. There is a harsh world out there of people looking to place blame.