Generally with exclusive contracts, the exclusivity refers to the images submitted.  Dreamstime and Fotolia both have these options... so you can upload a single exclusive image to those sites.  Macro (traditional) agencies also work this way and some require it, but it is always on a per image basis.. not EVERYTHING you produce.  The iStock exclusive deal is really very restrictive.  That said, some people (photographers mostly perhaps??) are doing very well as exclusive.