Some of the best seminar presentations I have seen have been blackboard based. But personally, I really like to be able to have both a blackboard and a video projector. I can use the projector for highlights, theorem statements, pictures that are too hard or time consuming to draw by hand. But for sketching out an argument or giving a detailed proof, the blackboard shines, for much the same reasons as outlined in other answers. Unfortunately, one can rarely rely on being able to do both at the same time, but I would like that to change.