I'm a real estate photographer, and I think this idea come to life would solve all my problems forever. With interior and exterior still photos there's almost always some color cast. Example - solar panels look blue instead of gray from sky reflection, ceilings and walls in rooms with hardwood floors and wood cabinets look orange/yellow.
Now imagine if I could go in, brush over the blue tinted solar panels I want to change to gray, and be able to drag the blue saturation slider down WITHOUT affecting the sky?! This would seriously be a god-send. I live in Camera Raw. As of now I use the sponge tool to desaturate one thing without affecting the other.