login - Showing users partial content to entice them to sign up?

I was trying to get one of my designers to check out a great question on Quora yesterday and he found to his annoyance that he had to sign up to read beyond the first answer

I was wondering if this was a poor design practice or if there is any research on whether showing some partial content to user actually entices users into signing up full time. Or would this be considered as a dark pattern which builds upon the expectation that people would be curious about something after seeing a small teaser

Note: Quora does try to sell it by saying that by having you signed in we can ensure we are showing you relevant content based upon your previous browsing information but wont this be a big turn off for first time users who want to just read about something.