The problem with blogs is that there are millions of them. How many blogs have you come across that seem to have run out of steam after a just a few posts? How many blogs contain poor quality content, are poorly put together, and, so on? As a media planner / media buyer you don't want to be associating your client's advertising with rubbish.
But the other side of the coin is that there are a lot of very good blogs out there too. Blogs that are written with real professionalism and enthusiasm. Blogs that offer the media planner (/ media buyer) good quality, traffic. The problem for the media planner is the time it takes to look, and research, for the right blogs. It's not easy. But if the media does his (/her research) well there are great opportunities to be enjoyed.