While I don't blog, anything that gets an academic writing and shaping their thoughts can be a good thing. And in the modern grant-writing process, it's absolutely better to get your ideas out there - it helps you to plant your flag, and it gets people thinking about your ideas. If your blog has followers, you get immediate feedback and critical analysis. And suddenly, you're recognized as the expert in that area.
Like it or not, at least in the US system, your grant application is not reviewed solely on its merits. The surest way to get a grant rejected is to have a really good idea that is completely unproven, and which a reviewer (who's probably reading 30 other applications that afternoon) will immediately be overly skeptical about. An application from a "recognized" expert will be far better received than someone who is just getting started. While most academics aren't going to know what a blog or twitter is, all it takes is for one reviewer in the room who does know to speak up and say "You know, this young guy with very few pubs has actually been deep into this for a while..."
But, yeah, there's a balance. If you should be in the lab, it's best to be in the lab, and you should absolutely not be live-blogging experimental results, no matter what.