The biggest iasue is that this tool is active in every context. If you activate it, forget it and get out of a component and into another it removea materials of the new one.
This is one of the rare occasions where I'd like a tool to change the layer of a face. It could then use color by layer instead of depending on a temp material. On exiting the tool, every face in the tool's layer coukd revert to layer0. A user could also isolate the ngon layer and see where the cukprits are.
Original report by me.
http://sketchucation.com/forums/viewtopic.php?f=397&t=66834&p=630155#p630116