I usually turn my phone and laptop off for the flight too, but it always bothered me that leaving them on would somehow end with all of us in a fiery wreck. It seemed like a plane was a bit too big of an investment to possibly lose simply because someone didn't turn their phone off.
Of course, by the same token, I always felt that it was such a small thing to ask that I never really had any issues turning it off when asked. While one cell phone may not have caused a significant amount of interference, I would imagine the cumulative interference of a hundred cell phones, fifty laptops and fifty tablets might be a different story entirely.
Def