Haven't dove with one since my certification OW class - couldnt wait to not dive with one. They get in the way and are only good for the first 2 inches or so of your dive. Below that, they breathe a little wet.
If theres ever been a need to swim at the surface for any distance its always on my back so dont need one for that. I would think the only time you might want to consider one is if you somehow had to wait in slightly choppy seas but even then I doubt i'd bring one.
If all you're doing is trying to save a few breaths from the tank before you go down, its likely not even enough to worry about and wont appreciably affect your bottom time. They get in the way when you try to grab for your bc inflation valve. They catch on things and generally make for a less pleasant experience all around. Keep in mind that they are absolutely useless for the majority of time you're in the water when you have scuba gear on. Not only useless, but they get in the way, so they
re worse than useless.
If you're snorkeling, bring a snorkel. Otherwise dont.
My 2 cents.
steve