I guess on the plus side he might be getting some thread cleanup cutting action with more teeth engaged. It might help with odd thread size or specific materials? But the thing is your lathe must have corresponding thread traversing TPI or MOD gear setup anyways, no different than single point SP threading
But I also see disadvantages:
- Unlike SP threading, you cannot disengage the threading with the tool tip safely in a thread relief groove because all the tap tool teeth are engaged. He therefore has no choice but to 1) quickly retract the tool back from cutting and clear of any enlarged boss then 2) disengage threading. If he messes this up just once, the entire thread section will be wiped out kaput whereas SP maybe just a fraction of the innermost thread.
- I have to think most of the cutting action is occurring on the very first tooth of the tap which he has probably ground flat. Aside from setup fiddling around & preventing the tap from turning, its probably not as strong a tool cross section as a dedicated vee form SP thread cutting tool. The tap will flex because its cantilevered & thinner cross sectional area, its not supported by the hole wall like in normal tapping mode
- he can only go from TS to HS direction, not reverse HS to TS direction like a SP tool starting out in a relief groove. So for the pucker factor combination of coarse thread (=fast traverse) + big shoulder you don't want to crash into, I'd take the HS to TS mode, therefore another point in favor of SP tool
Not taking away from alternate ways of doing things but I don't quite get the logic. I've heard of people power threading conventional internal threads on a tool post for specific applications, but that's not what he is doing here.