I recently came across this blog entry on Will Richardson's site. He describes a small aspect of why blogs are important - that being that they help to bring information to the public and to prevent "ownership" of said data. I feel this is a very good way to look at the subject, and a good reason for them to exist.
Many changes in history have come from the change in the availability of data - from the printing press making literature available to the masses, to information becoming freely available via the internet.
Therefore, I would say that a good blog is one that has value and can be used in the future to help other people. Many blogs can be useful for the opinions expressed about certain events in history, without being pure fact.