Well, I would assume people are working from a checkout. Patches from an outdated checkout simply would fail and that's fine by me.
Ok, but that's an extra barrier for contributions. Lots of people when asked for a patch just modify their distro in place and you can count yourself lucky if they send you a diff from a clean copy.
Well, either they're doing a small patch and uploading to Rietveld doesn't bring much anyway; or they're doing a large patch and it may not apply cleanly if they do it against the stable release. So I don't think there's a real problem here.
I used to get these frequently. I don't know what the experience of the current crop of core developers is though, so maybe my gut feelings here are outdated.
Most patches posted on the tracker are generated either by "SVN diff" or a DCVS equivalent (often hg). Regards Antoine.