Having PNMs all wear the same thing would make it tough for sisters to remember who was who, especially if you happen to meet several PNMs with the same first name. Being made to conform would also turn a lot of PNMs off, and would make it difficult for them to express their personalities through their outfits.
Of course, you notice how this doesn't apply to sisters. I remember going shopping with a bunch of my sisters for identical khakis and Keds (because heaven forbid we should have different shoes on!). One of the little things I enjoyed about being a rho chi was being able to wear whatever I wanted during rush
