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You Don’t Need to Use Airplane Mode on Airplanes
(gizmodo.com)
This is a most excellent place for technology news and articles.
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.
This is the primary reason I do it, although more for my battery life than the cell towers.