There's probably more skill than expertise in traditional PR. Most of my colleagues can make very good careers for themselves simply by learning from experience. (Theirs or their superiors'.) The trouble with interactive communications is that you couldn't survive simply on the opportunities and knowledge available to your predecessors five or ten years ago.
Having floated around for a couple of years on one of the lifeboats that just made it to a safe distance away from the dotcom shipwreck, Blogs represent, if not the sight of palm trees on the horizon, at the very least a cloud of buzzing insects around me and my famished fellow passengers.
In a field which requires regular, small doses of expertise the apparent ascendancy of micro-publishing is a major change in our professional circumstances. Books are too slow, magazine articles too narrow (and often rather rigid in style.) A blog that establishes itself as a knowledge aggregator for a particular field of expertise is the best way for us to refuel with information on the go, and to quickly convert this into the energy of insight.