I can definitely see why ABC classes exist (and I always through they were "anything but collie" classes meaning any sort of collie not just Borders?). In an agility system where you need WINS to graduate through levels, if you have a dog in the same height class as Border Collies (or heading dogs in NZ), there are ALWAYS going to be dogs that are faster than yours.
It does make more sense in places where you need wins to progress, although I would say that if that is the case, maybe all of of the dogs should be broken up into "classes", such as they do for Conformation (not the same ones), to put all of the dogs of similar build and running style together. Mixes could be tested to fit them into the most appropriate class.
Also, "Anything but a Collie" that includes Shelties, Aussies, and other similar herding breeds (along with similar mixes, I gather) also makes a lot more sense to me than singling out the Border Collies.
But in this country, progress does not depend upon "beating" anyone, and under those circumstances, I consider separating out the Border Collies - and only the Border Collies - to be flat out stereotyping and discrimination (I know that's strong language - but that my point of view and a Border Collie handler who does not fit the stereotype of having a Border Collie to be competitive). Separate out the Shelties and the Aussies, and whatever dogs tend to run fastest in the shortest heights, and I would object far less.