Is there any *customer-led* reason why one might not want to prefer customer routes over peer routes? (i.e. not "it saves me doing some backhaul as I can dump the traffic off to the customers other provider").
Yes. Suppose that I am "M", and I have two providers "A" and "B". The links M/A and M/B are expensive international links, much lower bandwidth than I would like, and prone to congestion. Further suppose that A is a customer of R, and B is a peer of R. For load balancing reasons, I would like R to send some of my traffic via A and some via B. Since I pay A and B for transit, and A pays R for transit, and A and B both agree to play along with my desire to load balance, it's reasonable for us to ask R to do this. From R's point of view, their customer A and their indirect customer M have asked them to treat peer routes (via B) and customer routes (via A) to destinations in M as being equivalent. --apb (Alan Barrett)