Blogs are mostly one-way marketing, they take what's already inside your company/brain/startup and push it out towards the world. That's the exact opposite of the direction that matters most.
OK, a blog is often useful since your customers will want you to communicate with them in some way. But worry about doing that effectively before you worry about incrementing some integer on a social networking site.
Well yeah, link-building and tweeting are also low-priority tasks compared to better understanding your customers and building something they want to pay for.
"they take what's already inside your company/brain/startup"
I agree.
There are actually valid reasons why companies want to keep information on themselves non-public to not give tips to the competition. Much of this has disappeared on the internet with the general openness. But only some of the rules of business have been re-written, not all of them. Of course if you want to release disinformation that the competition reads that has value, but obviously takes a bit of effort to be done correctly.
Walt Disney assembled Walt Disney World in Florida by setting up all sorts of intermediaries and even flying through various small airports so as not to tip off competition. He went to a great deal of trouble to keep his plans private (and of course there is Apple secrecy as well).
OK, a blog is often useful since your customers will want you to communicate with them in some way. But worry about doing that effectively before you worry about incrementing some integer on a social networking site.