Snorkel is Superior to Kickboard
I use a snorkel every workout for at least 600 meters, but NEVER for freestyle. Instead I use it as a substitute for a kickboard which I personally find one of the worst swimming "aids" ever invented.
Prior to last summer, I would breakup my freestyle laps using a kickboard. After only 25 yards my neck would ache, the lower back would cramp, and I would be out of breath from lifting my head every three seconds to breath. I never thought about kick technique, just about being uncomfortable and getting air.
I switched to a snorkel only after I used one on a Caribbean trip to surprise...snorkel. It was so relaxing to execute an easy kick in the same posture that I use to swim. Not only is my kicking technique now better but it's great for my "active" rest between freestyle lengths. I am able to kick on my front and both sides. Warning...don't over-rotate or you will end up with a mouthful of water.
I estimate I am about 50% faster using a snorkel rather than a kickboard. If you are trying to master a two-beat kick my guess is that a snorkel is a great aid. You don't have to stop and stand up. Just keep breathing and don't break your rhythm.
So hail to the snorkel (mine is a Finis) for improving the kick, but maybe not so much for swimming. By the way, kicking with the snorkel helped me to eliminate the dreaded scissors kick.