Do you mean that's the way they come, or that's your preference?
Lots of folks have talked about the benefits of mid length gas on a 16 inch barrel -- though I haven't shot one myself, I was lead to believe that if you ARE using a 16 inch barrel for 3 gun (which should be fine with optics -- MAYBE a disadvantage with irons) then midlength gas is what you want, no?
I think that it is very well known that the 14.5" barrel is best suited for a carbine gas system and the 16" is best suited for a middy gas. From there it's obviously a rifle gas system. A 16" middy will have a softer recoil impulse than a carbine system and an adjustable gas system on a rifle length upper is the bomb. I think I know the differences since I own all of the different combos (and then some )
Carbine gas systems were first designed for the the Colt Commando 10.5" guns, which predate the M-4 with its 14.5" barrel. When the military desided they wanted something between the 20" A2 and the Commando it was easier for Colt to adapt the existing gas system onto the new barrels then do the R&D for something new. Just because it was easier didn't mean it was best though. What resulted was a carbine that has been over-gased pretty much since it's inception. This is a big reason why the Army has been through half a dozen iterations of extractor setups on the M-4.