G2 caliper paint -
http://g2usa.com/
A couple of use cases for when caliper paint looks good to me.
1. If you have a front BBK, but OEM rears, it doesn't hurt to paint the rears to match the fronts. OEMs do this quite often.
2. Painting both the fronts and rears black so that they don't detract from a nice set of wheels.
Really though, it's your car, so do whatever makes you happy.