From Why Web Publishers Fear a Little Sharing:

“We use registration only where it makes sense to do so from a reader’s perspective,” explains Adrian Holovaty, lead developer of World Online, the Web division of The Lawrence Journal-World newspaper in Lawrence, Kan. “The classic example is on message boards and comment forms, where readers want to register so they can own the rights to their usernames — and, thus, their online reputations.”

Like I said: reputation matters.

[See also: I do not let things go.]