If there's something I daren't say under my real name I'd rather just not say it at all.
Well, that's true but that's only half of the story. The other half works the other way around.
Suppose you're applying for a job, your prospective employer googles your name and finds numerous posts from you, which were done throughout the (working) day? Even though there might be an entirely legitimate explanation for it, it's one more thing to explain in the interview.
Personally I value my privacy and try not to let my real name appear on the internet too much. The internet indexes everything and retains everything forever. I occasionally google my own name to show what shows up, and one of the things that comes back is a Usenet posting I did 20 years ago about some sort of complicated matrix calculation. When I read that post now I have no idea whatsoever what it's about anymore.
Fortunately I have a namesake (some 10-15 years younger) who doesn't covet his privacy all that much. He's the one doing the facebook pages and everything so I do have a plausible denial strategy in any case...
