So let's say you shoot the underside in single stage. There are a lot of nooks and crannies - places to potentially miss. If you don't catch them right away, blowing them in afterwards will leave overspray which you then have to sand and buff and "hopefully" have it blend in.

Now let's say you shoot the underside in B/C. You shoot the base and any missed spots are just blown in - no blending issues. Now you clearcoat it. If you do happen to miss any spots, blending the clearcoat is less labor intensive than blending a single stage - particularly if the color is a metallic. Furthermore, color match to the top side is perfect.

Is this all to save a couple bucks? Not worth it !!