I can't believe some people advocate diving without a snorkel... This is a basic safety requirement, and if an accident happens could make the difference between life and death!
* it is useful for shore entry and exit to avoid wasting air during surface swim
* it is useful for boat entry to avoid wasting air while waiting for everyone to be ready
* it is useful for boat exit to be more confortable while waiting to get back on the boat
* it is essential in choppy waters whenever you have to be at the surface
* it can save your life if for some reason you are lost at sea, or a long way from the boat/shore
* it can save somebody else's life during rescue operations
I strongly recommend getting a simple basic snorkel, with no purge valve and no splashguard. There are few good reasons for that:
* this allows you to perform emergency breathing through the snorkel during rescue operation, while still performing a comfortable pull
* if you are into freediving, a simple snorkel can be emptied simply by looking up and blowing gently in the last few meters of ascent
* purge valves fail; it has happened to me, and it's not fun swimming a few hundred meters while trying to block water from entering your snorkel with your thumb. I know a few people who have had the same problem
IMHO, a snorkel is a more important safety device than a BC: a BC will allow to float at the surface, but not to breathe easily, whereas with a snorkel, you can assume a position that allows your body to float naturally and avoid fatigue by letting your head rest while breathing easily.
Granted a snorkel attached to your mask can sometimes get trapped and cause mask flooding. So what? It's easy to readjust and clear you mask, and if a simple incident like this cause you to panick, you'd better get more practice in emergency underwater skills. In tricky situation where trim is essential, removing the snorkel from your mask can be useful, but make sure you always carry it with you.