What seemed to me initially to be a simple subject with a "personal preference" answer has turned into a bit more than that, and I sense a few misconceptions out there in the air...
First, a little history. When I started diving, no one that I knew or dove with had more than one tank, so a day's diving was one tank's worth. Air was truly precious. Our typical practice was to use only the snorkel on the surface - air was so scarce that we would snorkel over to directly above the site, hyperventilate a little and kick like hell for the bottom, only taking that first breath off the tank when we had to. Likewise, the switch to snorkel was immediate when reaching the surface. So, for me at least, the snorkel was an integral and important part of my early equipment - and I still carry one in open water today, where my role as an instructor often has me spending lots of time on the surface, and air is still precious.
Second, the presumption that you'll never need a snorkel on the surface in open water because of the way you dive is a bit naive.. statements like "a good diver should never let that happen" simply say "I've never had an "O" ring blow or had an out of air diver suck me dry at the end of my dive, or screwed up my navigation and ended up with a long, upcurrent swim to the boat - or, for that matter, had the boat screw up a drift dive pickup and leave me on my own for an hour."
All the arguments in the world one way or the other won't convince the die-hards, but an hour of sucking a little salt water every now and then will.
I'll continue to carry a snorkel in open water - at least in a pocket, ready to use.
Rick