The clearance light requirement has nothing to do with commercial or private use and if a DOT officer was ticketing srw trucks w/o lights he was uninformed. I would guess that was an unfounded rumor not fact.
Clearance lights are for the purpose of warning other motorists approaching in darkness that the vehicle is wider (or taller) than normal. Note clearance lights are used at the top rear and top front of trailers or tall box trucks as well.
I think dealer sales managers in snow country sometimes order 4wd srw trucks with clearance lights because they anticipate many of their trucks will end up with wide snow plows mounted.
Some just like the bright lights. Apparently it is a "trucker thing. "
I think you are right, heck I had West Coast Mirrors on my 1947 Ford PU in the 50's!