I think something like a Matte Material, that simply doesn't gets Rendered is much more interesting!
A render Region could be done very easy by placing a Object in front of the Camera,
with a totally Black non reflectiv Material so not only square regions would be possible!
I know this black are will steal resources too, but less than other materials and
a complex geometry it hides..
A Matte material could be used for Matte Purposes
want to change the material of an object in your scene?!
Just color all other objects with the matte Material, and Indigo will shoot
rays only on your couch...
(don't ask me how this will affect reflectiv materials etc.)
Such a Material could be also coded by "simply" allow priority for a material in %.
0 % Priority = Matte Material
You also could channel the power of Indigo into more complex geometry/material.
BTW
The Idea to add a poll to each [REQ] Thread is great!!!