If someone's website forces me to set up an account without me already being convinced there's enough benefit to me in return for my personal information, they're likely to get a signup for test@example.com with password "foo". (And if they then respond with "please click the confirmation link in the email we just sent", I'll sign up again with $sitename@$spare-domain-I-own.com and pick up the link from the spam filtered catchall account).

I probably do this at least once a month when there's hints something useful in a web forum I want to read, but I'm not (yet or ever) convinced I'll ever become a member of that forum's community...