One of the things I don't like about a snorkel is the increased dead space. Particularly if you are trying to swim against chop or current, you're going to get breathless a whole lot faster if you're trying to deal with the CO2 rebreathing that goes with a snorkel -- or at least, I do.
First, if you have a back inflate or wing, you probably can not fully inflate it and remain vertical. At some point, the wing will tend to push you forward and face down. So, you don't fully inflate. And this leaves you less than a comfortable amount out of the water.
This has not been my experience with any combination of plate, wing and tank that I use. I like my head WELL out of the water on the surface, so I often inflate my wing until the OPV bubbles. I can lie on my back, lean a little back, sit vertical, or swim leaning slightly forward, or with my face in the water. I don't have any problem with any of those positions. They are all a bit easier in very cold water, where I have a ton of ballast around my waist, but I can manage them all in warm water with minimal weigh as well. I think the crotch strap makes a huge difference. If the flotation bladder rides up high onto your back, I could see ending up pushed forward.
In Puget Sound we have a lot of reasons not to like snorkels. We rarely have significant chop, and the water is too opaque most of the time to see anything while snorkeling on the surface. Our charters are mostly live boat, so you don't have long swims to return to the vessel.
The only time I might see a snorkel being useful is if I get separated from the boat in water conditions that are so rough that dropping my weights and inflating my BC can't get my head out of water, and I'm drifting longer than my remaining air supply (which is usually substantial) will support me. That's so many things going wrong on the same day, I think it's pretty unlikely to occur. I don't feel motivated to carry a snorkel with me on several thousand dives where I don't need it, in case I hit the one day where I do. It's my risk assessment; everybody does this differently.