This seems like one of those things that gets taught in OW classes just because it's always been taught that way and the agencies aren't interested in rethinking it, they are slow on changing their gospel. As with most gear there isn't one right answer that is best for all situations, carrying or wearing a snorkel included.
I keep one in my pocket and I think it's a good thing to have tucked away just in case, but I've yet to have a serious need for it. My mouth works just fine for breathing on the surface, no extra equipment needed. My reg works best of all as I normally have lots of air left. If there's waves and I have air I'm using the reg. If you have a longish surface swim and need to conserve air a snorkels good for that, unless you swim on your back.
One common safety argument against wearing a snorkel is that it's an unneccessary entanglement risk. That's more true in some environments and not much of a concern in others.
Another thing is that it might make your mask leak, at the very least a this is a nuisance and I suppose it could be argued that it would increase some people's stress and task loading. Snorkels when diving are a PITA pulling on your mask and generally getting in the way. Being comfy in the water is also a factor in being safe.
Wearing a snorkel in a strong current isn't a good thing, you don't need a sail trying to pull off your mask.
If you ever go to Stingray City in Grand Cayman, they tell you not to wear a snorkel else the rays will have a grand time using it to pry off your mask.
I think the odds of wearing a snorkel causing you a serious problem are extremely low for most people on average open water dives. I think the odds of a disaster on the surface because you're not wearing one all the time are also incredibly low.