To let customers decide priorities in your backbone is a bad idea, but I don't think that's the issue here. Assuming the customer's link to the network to be the primary bottleneck; there's nothing wrong with giving customers the ability to prioritise traffic on their link, provided that your access-equipment is able to handle queueing etc (given fool-proof mechanisms that enable self-service and keep your NOC out of the loop of course;).
Precisely! In today's world, lots of router configuration is not done manually by anybody. There is an OSS system that applies rules to what changes will and will not be done and when they will be done. Since QoS works by degrading the quality of service for some streams of packets in a congestion scenario and since congestion scenarios are most common on end customer links, it makes sense to let the end customers fiddle with the QoS settings in both directions on their link. Of course, any incoming packet markings should be discarded or ignored once the packets pass the provider's edge router. This is possible today without any special support from router vendors. It relies entirely on operational support systems such as web servers, databases and remote control servers. QoS is for customers, not for network operators! --Michael Dillon