It can be unnerving to be on the surface in heavy chop. I don't think a snorkel makes things much better, since the water will break over the snorkel, too, if the chop is rough. If you are calm, it's not too difficult to time your breathing so that you don't inhale when you have your face in the water, but if you are frightened by the conditions, you may not be able to do this. And if you are trying to do a shore exit in rough water and get rolled, it's a VERY unpleasant experience.
In rough or challenging conditions, keeping the scuba regulator in your mouth at the surface is generally desirable. A big deal is when you are exiting the water onto a diesel exhaust boat. With a mask on to remove your ability to smell, you can easily inhale several very strong breaths of diesel contaminated exhaust and you don't know this until you taste it and by then you will be sick (and puking) within a minute or so.
Keeping it in your mouth while climbing out also, prevents it from being tangled on the ladder on exit. I have seen several instances where people come up the ladder with the reg out and they just rip the (plastic) second stage right off the hose. And of course the potential for falling back into the water on the climb is another reason to keep it in your mouth.
When I was taught to dive (40 yrs ago) my peers expected you to switch to a snorkel on the surface because only pussies need to suck on a tank at the surface, plus they had been taught with j-valves and no pressure gauges, so the tank was pretty empty on the surface anyway. Now I generally keep it in, while waiting for a pick up, because I am diving nitrox and figure it helps with the off gassing.
Another exception, when diving in very cold water and swimming in on the surface, I much prefer to breath warmer, moist atmospheric air from my snorkel than chilled, dry air from the tank. I swear I warm up faster when using a snorkel.
However, all that being said.... A snorkel at the surface in rough sea conditions is a tremendous advantage (if you are out of air or very low on air and want to save it for maybe ducking down to 60 feet if a ship tries to run you over). I feel that if a person finds using a snorkel more difficult than breathing from the mouth in rough water.. they simply do NOT know how to use a snorkel properly and how to control their airway effectively.
We sometimes freedive in rough conditions, with whitecaps crashing over our heads. It is actually surprising how easy it is to use a snorkel in these conditions, IF you know how to do it. I honestly believe that for a scuba diver to be safe, they should be able to survive with an inflated BC and breathing through a snorkel for a long, long time, in most any reasonable sea state.