A couple things not mentioned here.......
First, I think taking one along on typical sport dives is a good idea.
The wife never wore one on her mask from day one but we always had them stowed upside down alongside our back mounted BCDs. You could pull it out and even shove it under your mask strap for a long surface swim. I still believe keeping your regulator in and mask on is preferred on the surface waiting for boat pick up, etc.
Second, I think today's snorkels are simply awful. Any flex mouthpiece will fold over in a semi-strong current. Ones with huge extra drain valves resembling nuclear cooling towers have so much drag above and below water. If you free dive down hey wobble and beat you in the side of your head
Third, foldable snorkels while a good idea for mass only scuba and space saving will collapse too often for any serious surface swimming. Very few maintain any rigidity.
A simple curved "J" type snorkel like free divers use is a better solution IMHO. It hugs the head better and some folks may elect to keep them on their masks (I don't) and is an inexpensive solution. We all did back in the day simply tucking the mouthpiece under your chin.
On a Cozumel boat one time a huge pod of dolphins started following us surfing in the boat's wake. We all excitedly asked if we could jump in for a look see. While others were screwing around I grabbed my snorkel behind my BCD wing, shoved in under my mask strap and was off the boat as soon as the Captain hit neutral
I enjoyed a few minutes free diving down, spinning with them while they checked out this strange creature. The group barely got off the boat and the dolphins moved on.
Just one old guy's opinion
David Haas