"However I do think some Web/REST advocates need to look around and realize what's happening on the Web instead of arguing from an 'ideal' or 'theoretical' perspective."
The Web advocates need to realize what's happening on the Web??
Okay, we all know some resources have broken content-type headers when you retrieve them. Others don't. Some clients never use content-type correctly, others do and many use heuristics. That's okay. Just do your homework - and part of that is to read the TAG finding and use it where appropriate - and be a good web citizen. That's simply practical advice based on practice. The "don't use content-type" is a theory that is unproven in practice.
Update - it looks like Dare found breakage in Cookies as well, but after further investigation it turned out to be a problem with the data returned by the server. But if the bloggers using RSS can't correctly control this Cookie response header - would we have no choice but to drop support for that header as well, as suggested for the Content-Type header? I mean, the theory of Cookies is all well and good, but if you look around to what the major players like MSN are actually doing, who are we to stick with something broken as designed merely because of theory?