You're just completing another awesome dive. You're finishing your safety stop thinking about that cold one in the cooler. As you breach the surface with 500 psi in your tank you do sort of a pirouette and nothing, no boat. Seas kicked up a little bit so you try and time your search for the boat on the crest of the waves, still nothing. Sharks begin to investigate what this thing is making all of this racket on the surface, finally your tank is empty and now you need to tread water and keep your mouth above the surface. How long can you keep this up before you aspirate water?
Happened to me in April of this year. The one thing I wished I had was my snorkel. After twenty minutes I was found and if I had my snorkel I would have been much better off. Fortunately I had two DSMB's and could use one as a extra horizontal flotation device that I hung onto under my arms to help keep me above the waves and make it easier to keep the other one upright. Twenty minutes isn't very long, but it could have turned into hours or longer.
We had another diver in our area report on another board this year of an incident where he did a 4 hour float. The one thing he was thankful for was his snorkel. Despite his jaw being sore, IIRC, he said he didn't know if he could have survived without it. Despite having it he still managed to drink/aspirate a ton of sea water.
So, you really have to ask yourself, is there a possibility that you could find yourself in that position after a dive? If so, at a minimum you should carry a pocket snorkel. If you dive in caves, spring, caverns or tourist cattle boats in large groups then it's probably not needed, especially the former. But this is a question that has to be answered, because many will say they are useless, but they could be doing dives that have a remote chance of being separated from the boat, whereas your dives may have a higher possibility and you just took the advice from someone on the internet who dives in quarries.
For me, once you start wearing a good low profile snorkel in the proper position (toward the back of your ear and not by your cheek) you never even notice it is there.
Hope this helps.