Personally, I’m not much of a fan of snorkels. However, as a PADI dive instructor, I’m required to wear one when teaching or acting in any sort of official capacity so I always have one packed and ready to go in my dive kit. Out of habit it ends up going with me on non-teaching dives as well.
I mostly swim face-up in the water and so I never really even put the snorkel in my mouth. In full equipment, I’ve always found the face-up puts my body in an optimized position to get the most power out of each fin kick. Even in rough seas, I still prefer this style. Thinking back, I don’t remember a time when I’ve ever used a snorkel during a surface swim so part of my view on snorkels originates from my belief that a diver should shed any gear he doesn’t plan to use on the dive (except emergency equipment such as signaling devices, etc). I mean, I wouldn’t carry a wreck reel on my belt to do a drift dive along the reef in Cozumel even though it’s a useful (and even required) device for other types of dives.
Divester provides a good counter-argument though. It might not change my mind about what I prefer but it’s a good read in terms of the arguments for wearing (and using) your snorkel. Well worth the read.