What does r_stereo do and why is it off by default?
If it does what I think it does...
It doesn't work in QL.
It's supposed to add an anaglyph effect for use with 3d glasses and other such devices.
Would be controlled with:
..and the effect would look something like this (xonotic):
The impression I get from random google searches is that it was always kind of a buggy/gimmicky feature. Like even when it works people aren't that impressed.
Probably nowadays with Oculus Rift going and all we might see some better 3d implementations in games but for now there is still very little interest. It's one of those things where unless the implementation (both hardware and software) is absolutely stellar then it's almost always better just not to bother.