People who blog have a known level of transparency. You can see what they think, what they're up to, and, often, how skilled they are. It shocks me when someone who clearly spends a lot of time online and professes about how skilled they are has no blog, journal, or even a personal Web site.
Not everyone has time to update a blog, I know, but the bare minimum of their achievements would do. I would find it hard to hire someone who wasn't a blogger for something like FeedDigest or an online marketing or PR style role because they're clearly not making the effort. It'd be like hiring someone who had no résumé, references, or anything to say about themselves.