Some of us just think we have enough crap to carry with the doubles/SM with all the regs, and the stages with their regs, and the deco bottles with their regs (or the CCR and bailouts with regs), and the reels, and the lights, and the cutters, and the SMBs, and the strobes, and the goody bags, and the wreck tools, and the DPVs, and the argon systems, and the drysuit, and the EPIRB, and the spare mask, and the wetnotes, and the compasses, and the computer/BT, and the spare computer/BT. Did I miss anything? Almost certainly.
A snorkel isn't so you can breathe while swimming, it's so you can simultaneously see underneath you and breathe while swimming. If I want to see what's under the water and I have dive gear on, I'm going under to see it. It also makes breathing while maintaining a front crawl stroke somewhat easier, but I don't see too many divers doing their surface swims using that approach. If a diver has a long surface swim where they want to see the pretty fish but not use gas, a snorkel is probably a fine tool to bring…but I usually see it being hocked as a safety item with some gas-saving convenience benefits.
I don't see the appeal of a snorkel for rough water, and I've been out surface swimming in seas where my buddy was a good 15'-20' above me on the crest of the rollers while I was down in the trough, both of us trying to make sure we didn't land on each other. If you're adrift or even just swimming for a long time in rough seas, you want to keep your eyes and ears out of the water as much as possible to see what's happening and adjust your position as needed. IMO, if you've got a BCD or a wing or anything else you can inflate, the best way to manage that while making sure you don't wind up with empty lungs right as you get dropped into the spin cycle... is to be on your back, not with your face in the water breathing through a straw. It'd also suck to miss spotting a potential rescuer because you had your face in the water, breathing through your trusty snorkel.