I find a snorkel to be very useful for, well, snorkeling. Which I like. It's great, you don't have that big heavy air tank, you don't run out of air, and there's lots to see.
A good hour of snorkeling is better than a bad hour of diving. I was snorkeling with my wife (non-diver) at the Christ of the Abyss statue in the Key Largo area last summer while my kids were scuba diving from the same boat. They ended up being grouped with a DM and two other divers of doubtful skill and experience who ran out of air early and had to return to the boat utilizing the Surface Swim Of Shame. My kids were displeased and I am quite sure I had a better time than they did.
As for what kind of snorkel, I find that I am indifferent and can have a really good time in the water with whatever snorkel is at hand. I guess I would say my favorite is an XS scuba "metro" snorkel that has a discharge valve so it is easy to blast clear, but that does not have the float valve typical of "dry" snorkels. It has a corrugated flexible section.
Metro
Now, when scuba diving, I rarely bring a snorkel. There are a few situations that call for it for reasons of safety in the event of an unexpected amount of time at the surface.
I see snorkeling as being one of the skills that is part of being an all-around waterman, like being able to swim, row, sail, fish, etc.