It depends.
If I'm doing a shore dive with a lengthy surface swim to the dive site, I'll wear and use one.
Most of my dives are boat dives, either for spearfishing or as a DM.
For spearfishing, I'll carry but not wear a snorkel in case I have a long swim back to the boat (haven't deployed it yet).
As a DM, it's counterproductive to wear one: when I'm at the surface, my face needs to be up watching the divers; when I tow a diver, my head is mostly up; the snorkel creates an entanglement risk when I'm in proximity to trail lines, granny lines, and divers struggling to navigate along them.
PADI's AOW course book suggests putting a snorkel in your mouth after you surface. The rationale offered is that if your feet get bumped or entangled and you look down to see what's going on, it's better to already have the snorkel already in your mouth. I don't find that persuasive. I suppose a snorkel is better than nothing, but I'd prefer to see customers keep their masks on and regs in their mouth the whole time they are not in the boat--unless it's absolutely flat calm.
I think boat divers who try to conserve air by using a snorkel for the swim to the descent line are either a) overestimating the air they're saving, or b) already destined to have short dives.