Whether a blogger is writing to make a living, share expertise or fill their spare time is not the critical distinction to make. These motivations do not necessarily have bearing on the most important factors: influence and credibility.
The real challenge is to identify the most important blogs to target. This is a complex process, but knowing your bloggers depends on factors such as being able to weight blogs according to their audience and the wider credibility of the blogger in the industry. Too often people are splashing around among blogs that are either irrelevant or do little more than regurgitate other people's news.
You are right to draw attention to the importance of understanding who actually writes the blogs, but this should only be once the right blog has been identified as a relevant and powerful outlet.
Will Arnold, director, Apollo Research