Years ago Scuba was quiet archaic...The basic system was a tank, backpack, weightbelt & double hose regulator...No spg, no bc, computer etc.
Those days...long surface swims were very arduous, when you surfaced most likely you had little or no air the tank and it took a lot of effort to keep your head above water so the snorkel was a major safety item to allow a diver to lay prone in the water and still breathe. Your other optons were to start ditching gear.
Recent designs in BC's keep you quite high and dry and following rules, the diver should have plenty of residual air for any surface activity.
One thing to consider is using a back inflate system...they do tend to float you face down at the surface...this would rationalize using a snorkel if you might expect a long surface swim.
Joe