I think it's still worth doing so your phone isn't rapidly trying to connect and disconnect from nearby cell towers as you ascend and descend. Maybe there's already protections in place for this case but it makes sense that it could add a lot of unnecessary strain on certain cell towers.
Based on what I last heard, this is the actual reason nowdays. A plane full of people, each with a cellphone trying to connect to cell tower at high speed basically ends up with DoS.
This may also mean someone might not be able to make emergency phone call, so do turn it off.
I thought that's what the Pico cells are for - each plane has a mini phone tower in it so phones happily connect to that instead of looking for ground stations