In many cases even a seemingly small change will actually affect the entire render, if not necessarily in a visible way. In this case the atmosphere samples change *would* affect the entire render and such a system would be useless. Although there are situations where it could save a lot of time, actually implementing it could be very difficult, especially given the effects of Global Illumination. If it ever were to be implemented it seems like an advanced feature that would not be seen until later releases, TG3 perhaps.
- Oshyan