Indeed; as an user, I don't want the site to decide what quality to serve me based on probing my device. It'll only lead to the usual abuse. I want to specify I want a "lightweight" version or "full experience" version, and have the page deliver an appropriate one on demand.
I remember when websites used to have "[fast internet]" or "[slow internet]" buttons that you could use to choose if you wanted flash or not. Even though I had a high-speed, I chose slow because the site would load faster.
