1) People who are active users of the site, for whom registration isn't an issue. There really is a lot of value being signed in if you're actively using the site, with notifications, drafts, credits, voting preferences weighting what you see, etc.
2) People who come to Quora for a specific answer to something via SERP. They generally want one answer, and it's probably reasonable to ask them to log in if they're going to use the rest of the site. If 50% more bounce due to reg-requested than not, it still doesn't hurt things too much.
The case where registration links suck are when you're writing something and send a link yourself to other people. They added the "share this elsewhere" functionality, but I hate it when I write something, post it on FB/twitter/etc., and then people can't read it due to registration wall.
There's probably a much better way to do registration-requested than what they're doing now; all inbound links are open, but some kind of lightweight account created, and then a prompt to register including your existing browsing history, upvotes, etc. (otherwise they're trashed) after you navigate. Maybe make it a brief status area thing to start, then an interstitial, then a modal, etc. Give the user a clear benefit for registering even after 3-4 pages, rather than making it a binary wall.
