In my blog I have published a post with some my thoughts about the exclusivity question (per images and full photographer exclusivity) that has became even more actual after the recent announce Serban made here at Dreamstime..
First point to be made clear: to go exclusive as a photographer is not an option for me now. I speak mostly about exclusivity per images.
In theory, the strategy to have exclusive images could be eventually a benefit, as you don't compete with yourself, but is hard to came up with lots of (good) different images for different agencies.
You could have on Dreamstime images of people in business for example and in another agency images of wild animals or travel. This could allow you to have more revenues for every single image, if... The problem in fact is that mostly agencies reward well the exclusive photographer, not the single images.
I know this is a subject that could spring a good debate and is ultimately a personal choice, but from a business point of view, seems to me that is not convenient to have exclusive photos nowhere, at this point (dreamstime assignment aside! :-)