Just being devil's advocate here since public use of cell phones are kind of annoying. But...
I think the electronic interference that Captain Paul mentioned comes into play in really close proximity. It's just like if I have my cell phone sitting next to a radio or near my monitor (non-LCD) or some other piece of electronics when a call comes in. Move it away even a foot or two and it doesn't happen.
I think for a passenger, the effect on the instrumentation would be almost non-existent but I guess they're just being extra safe. Then again, if you know anybody in the aerospace industry and know what cost/benefit decisions airlines and plane manufacturers make that put your life at potential risk... seriously. Some person somewhere crunches numbers to decide if something added or excluded from a plane will be cheaper than the cost of an airline disaster. Sadly, that's business and that's life.
-Kevin S.

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