While I agree they are not a complete representation of a person, I think they are a _very good_ representation.

Everything you described (withholding information, restricting yourself to topics, know when to shut up), I'd argue, are actions you do all day with the folks you interact with. You're always thinking, "Who are these people, what do they want, and what should I tell them?' Based on who your're talking to, you give them your work_face or your friend_face or your home_face...

Weblogs, in my opinion, are no different. The process by which you filter what you say is part of how you are perceived. I think folks wrestle with this with weblogs... the immediate example that comes to mind is Dave Hyatt of Safari fame who split his weblog down the middle... PERSONAL and SAFARI.