1) One could be wrong.
That is, preliminary work has not been completed drafted, revised, exposed to feedback and then submitted to a review process. So you could be wrong, and that would be bad. The recommendation then would be to only blog if one is confident about the robustness of the findings.
2) It could make it harder to get the research published in an academic outlet.
How so? Several ways. First, reviewers might know who you are, and that taints the blind review process. But conference papers are already out there for reviewers to find. Second, journals want original research that has not yet been published. If you "publish" it via a blogpost, some editors and reviewers might think that the research is already published. I think this is a risk. I do think that most reasonable scholars would consider a blog post not to be the same thing as an article so that there is no double publication problem. Given the brevity of most blog posts, they can only address a small piece of an article or book. Yes, the findings might be less sexy if they are already out there some where, but most scholars would expect an article eventually that provides the social science that buttresses the claims of the piece. Third, your idea might get picked up by someone else and published faster. Getting scooped can be painful and even career threatening. So, I am less sure of recommending to junior faculty that they blog stuff that is not ready for submission to journals. Senior faculty, having tenure and all that, face far less damage about getting scooped.
In my email to my friend, I indicated something else. That if the research could help clarify something right now about an important issue or crisis, then one is almost obligated to share one's knowledge. I do tend to think that our job as professors is to create and disseminate a better understanding of the world ... so that we can help foster progress and improve the world. If we can do something right now, then we should even if there are some risks to ourselves.* Academic freedom (which sometimes may be in peril) is there for exactly this reason--to give us the room to present our findings even if they are uncomfortable truths. Especially. This is what tenure is for, right?
So, I have to conclude that one ought to blog or op-ed or be interviewed if one's current research has important implications even if it has not made it through the publication process.
* How can I ask the cops to assume some risk by being restrained if I cannot assume some professional risk myself?