If you're passionate about a subject, you're going to have strong opinions about said subject. If you voice/publish those opinions, some people are going to hate you. Perhaps a potential employer was set to hire you until they read your blog post in which you were eloquently critical about offshoring, for example.

Or worse, you expressed an off-hand opinion about some social issue back in 2006 in an otherwise technical blog post that the wrong person stumbled upon and now the world is going to cave in on you.

I would love to write more, but censoring myself for the sake of future employers, psychos, etc. tends to make anything I do produce come off as bland and uninteresting.