I was just talking to a writer friend of mine who said she doesn't blog because that would take time away from her writing. That's true, but that's assuming she writes a lot, and regularly.
Some writers I know do get up in the morning, sit down, and put in a good day's work.
Most writers I know slack. Everyday life distracts us and derails our trains of thought. We're certain we can't write coherently unless we have a certain uninterrupted block of time in which to write.
So what does this have to do with blogging?
I've found that blogging rhymes with unclogging for a reason. Well, actually, there is no reason, it just does. But anyway, blogging--regular blogging--blogging on a schedule--has gotten me in the habit of writing every day. It's gotten me in the habit of being able to organize thoughts quickly and get them down and get them out. It's gotten me in the habit of thinking of writing as packets of thought. In fiction, the packets just connect to each other to form a story arc.
And I love reading other people's blogs. I like sampling their writing and getting peeks into their worlds--real or created.
So, in the end, I have to say that blogs are definitely a use of a writer's time, but definitely not a waste of a writer's time.