I will readily concede the point that these can be useful in "SKILLED" hands.  There is a great potential to ruin a good blade on one of these, by allowing the heat to build up in the tools edge, thus ruining the temper.  Over grinding will shorten the life of a tool, and grinding the wrong angle will end the useful life of a tool.
 
Be sure you know what you are doing before you destroy an otherwise perfectly useful tool.