Here's essays from Tom Morris (another philosopher):
http://en.wikipedia.org/wiki/User:Tom_Morris/The_Reliability_Delusion http://en.wikipedia.org/wiki/User:Tom_Morris/The_Definition_Delusion
While some editors do tend to argue binary options over sources, in general this is not the case (and if you are observing it as so, it's probably one of the battlefield areas where such things do occur).
WP:RS has always struck me as being quite carefully worded to suggest factors of a source that editors should critically consider in determining reliability (publisher, author, content).
Take for example the Daily Mail, which we quite often discuss in relation to BLP articles. This is treated as potentially reliable media source as it is published and edited, on the other hand it has a reputation for tabloid sensationalism so naturally it's not the best of sources to use in biographical articles on its own.
There are other examples too. For example Torrent Freak is considered fairly unreliable as a source, but specifically for factual information about the Torrent community (and associated) it is explicitly considered acceptable. TechCrunch is considered fairly reliable for technology news - but has a recognised tendency for sensationalism which requires caution.
In the "Context sensitivity" portion of that essay Morris makes some good suggestions - but I see that approach taken literally all the time... sure in some areas (and for some editors) the idea of a reliable source is very absolute. But largely this is not the case. In contentious areas it is applied much more uncritically, of course, as all policies are - which is why you will see much more binary classification in those areas.
:)
Tom