Why? I use the same name wherever I can. I just looked at your home page.
As do I, ordinarily. For me it's mostly a matter of laziness - I just get so tired of trying to remember which username goes with which website, and what the password is - for passwords, at least, I came up with an easy-to-remember, but very idiosyncratic, algorithm for generating passwords, so that at the very least I can usually work out a forgotten password before I hit the account-lockout limit.