I don't use a snorkel although perhaps if I was active safety on line divers perhaps I would use one. I have had experienced spearos tell me the snorkel is better safety in the case you are incapacitated but to me this contigency isn't worth breathing through the thing, plus added drag and yet another piece of gear to get tangled in the kelp. I have tried to use them in tropical conditions as well but didn't care for it there either. Some authorities claim a sorkel in the mouth is extremely dangerous if blackout occurs while others (Terry Maas for one) claim there is no added danger.
I think some guys do stuff like run on a treadmill while wearing a snorkel to get used to them.
Breathing up head out should not reduce dive response, if anything it might improve it since the temperature differential would be greater when you submerge... Although wearing a mask in typical conditions I doubt it would make a measurable difference. You could argue that the snorkel is an energy savings but in my 5mm wetsuit I float like a cork at the surface, and I like to know what is going on out there as well.
On an energy/efficiency level I suppose it is like seals and dolphins.. Seals don't cover much lateral distance on the surface. They surface, catch their breath, then dive/move again. Dolphins seem to have no problem covering lateral distance very near the surface but they have blowholes up top (snorkel).