I put the dividing line at 12,000 lbs. towing load for needing a diesel over a gas engine. Download the towing guides from the truck manufacturers and you will find current tow load specifications for their trucks based on the engine and the gears. Where people usually and often screw up is in getting a gas engine with 3.42 or even higher gears. With the diesels this is harder to do as often they only come with 3.73 gears so no way for the buyer to make a mistake.
A diesel helps with acceleration and with grade control with the exhaust brake found with most diesel engines (for some it has been an option). Going over the Rocky Mountains with a 13,000 lb. 5th wheel and a diesel is the obvious choice. Going over hilly terrain with a 7,000 lb. trailer and a gas engine with 4.11 gears is all that is needed and diesel is a luxury purchase.
Nothing wrong as such with buying a diesel anymore than a guy buying a Corvette with 500 horsepower to commuter to work. Both provide a measure of ego gratification and if you don't mind the extra expense and the waste then go for it. But don't pretend it is necessary and logical.