Your buddy is right technically, but the difference is very small. I doubt you will ever see a situation that will prove him right, so he wont ever get the glory.
To anyone who now thinks I am an idiot, think about the concept logically.
The way the engine bay is shaped, the air has to flow in the radiator, and how does it exit? Some probably goes out the fender well (near the frame to fender gap) probably a little through the gap near the top of the firewall in the big formed holes in the fender, but I think the vast majority has to hit the firewall, and go down under the vehicle.
An air dam in its design moves more air around the vehicle rather than under it, so that would surely lower air pressure under the vehicle. Less air pressure in the air pocket under the vehicle, less resistance for the hot air in the engine bay to exit underneath, so you should be able to get more air rammed through the radiator.
Also, the above post about a 8 lug 99 3/4 ton not having one from new, that is because no 8 lug rigs ever got an air dam standard, and I dont even think it was an option.
I have never owned a 6 lug 2500, so someone that does and had it since new will need to tell us if they had them or not.
Like said above, the 6.5L diesel had a duct to seal the bumper to the core support so the air from the bumper nostrils would flow to the radiator, IDK if the duct is still available, I try to snarf them up whenever i find one in a junkyard.
As far as use, most of my GMT400s are the 6.5L diesel, and when I am over the factory tow ratings by quite a bit pulling some of the big passes out west (Around 21,000 combination pulling Donner Pass, around 22K pulling Vail Pass) I like to have the maximum cooling I can get, so I run the air dams. After installing, I have noticed at speed before the fan kicks in and with plenty of natural ram air before I drop speed, it will hold temp longer before warming up, so I think it does make a difference.
Also I dont like staring at a-arms and frame components and love mpg so that is another small reason why I run them. They are dirt cheap through GM, and I get the clip nuts and bolts off a tore up airdam on a half ton in the junkyard so I have all the nice factory mounting hardware to make me feel all warm and fuzzy.
With air dams I can clear 20 mpg with most of my GMT400s, IMO saying their incapable of good mpgs is just a lame excuse for someone not willing to try, or always making mods that are a detriment to mpgs. "Look at my new visor, 4.56 gears, and I got rid of my 4L60E and put in a turbo 400 in. My mpgs are not very good, but its a GMT400, she ain't never gonna get good fuel economy"