I frequently edit certain materials and only want to view their effects so it would be very useful if only those parts o the render with these materials get rerendered and the frame buffer for the rest stays the same. This could, for instance, be an addition to Render selected drop-down.
Maybe you could add an option to only render objects or places where the material was updated. So when I change a material in 3dsmax slate material editor, only the objects (or better yet parts of objects) with that changed material get re-rendered (the rest of the framebuffer can stay the same). This should of course also work for multi-subobject materials.