. . . users prinicipally don't need to know all the details of network their experiece rides on - just their experience.

At the same time, I get nervous when the policy debate superstructure gets too far removed from the nitty-gritty of the infrastructure engineering. So, fwiw, here's a well-known vendor's perspective: Supporting Differentiated Service Classes in Large IP Networks, Juniper Networks, 2001.

Executive Summary This white paper is the introduction to a series of papers published by Juniper Networks, Inc. that describe the support of differentiated service classes in large IP networks. This overview presents the motivations for deploying multiple service classes, the fundamentals of statistical multiplexing, and the impact of statistical multiplexing on the quality of service delivered by a network in terms of packet throughput, delay, jitter, and loss. We also provide a brief history of the various approaches that have been proposed to support differentiated service classes, a description of the IETF DiffServ architecture, and general observations about what you can expect from the deployment of multiple service classes in your network. [...]

I'm afraid this paper is somewhat dated, but in some of their more recent marketing literature, Juniper is still recommending this intro. (& Btw, please note that I don't necessarily endorse the pov presented in this paper—I'm not selling equipment! It's just that I do think recognizing different perspectives is valuable.)