Not sure what you mean by "Regular". ULSD is less than 15ppm sulphur. The previous LSD was "less than 500ppm" Prior to that was 3000ppm.
You are just putting more wear on the cat--more soot to burn off, more ash to contend with.
Over the long haul not a good idea, a few tankfuls? no problem.

But I don't see the real upside. Non-taxed off-road fuel???? (hmmmm?) Red dye?