because I always have my snorkel mounted on my mask. I was taught that way, and old habits are hard to break, good ones and bad ones.
I like many of the opinions given here, and I particularly agree with Scubascott and PhotoTJ.
I like to use the snorkel at the surface to conserve air before the descent. Some of that is habit, some of it is trying to squeeze every extra second more of bottom time out of a tank without compromising saftey margins. Many of my dives are in Monterey Bay where 3 foot swells are considered "calm seas". I go the extra wuss level by having a "dry" snorkel, but I haven't "swallowed a gobful of blue water" yet.
I have thought about removing the snorkel from my mask, and carrying one in a pocket, but it hasn't happened yet.
(Anecdotal info) By the way, there are places in Southern California where you can be cited if you dive without a snorkel, so the foldable snorkels are very popular there.
Wristshot