I agree that observing aircraft is really a risky business for a U-boat and should probably be avoided.
My Faroes example was not that thought out, just an example on why BdU would have found airplane type info interesting. We don't know what the U-boat commanders' exact procedure with airplanes was, but they did try to identify the type of the plane and wrote the info down for BdU to assess. Take a look at this war diary:
http://www.uboatarchive.net/KTB628-3.htm
It identifies a probable Whitney, Catalina, land-based airplane, flying boat, 4-engined plane and probable own planes. I would suspect they would have done the identifying bit after the danger had passed from an aircraft recognition manual of some sorts. In RL observing an airplane through the periscope would indeed be too dangerous for my taste as well.
It's a "to each his own" thing, like you said. The different playing styles and mods allow very different approaches to the game.
