I think that if an inadvertently left-on cell phone could truly disrupt an airplane we'd see MUCH stricter enforcement. Otherwise you're implying a band of terrorists playing Angry Birds in all-radios-on mode stands a significant chance of downing an airliner...
If cell phone interference crashed planes, no plane would make it off the ground. I doubt there has been a passenger flight in the last ten years that took off without at least one cell phone operating in the cabin.
tl;dr Cellphones and other electronics are more of a risk than you think
They aren't.
Who are you going to believe, some engineers writing in a magazine or a massive experiment conducted over a period of 20 years involving every single passenger flight? Tens of millions of flights per year. Cell phones on every single one of them. No crashes due to cell phones.
Cell phones do not cause unacceptable interference with airplanes. Q.E.D.
Things are not either safe or unsafe. It all depends on trade-offs, and what risks we can accept in exchange of other values, such as comfort.
The article mentionds, though, two worrying things such as the bias in investigations and ongoing GPS technology usage increase.
I would also stress that it _might be not_ linear. Combined risk of few hundrends of turned on cell phones is not a mere multiplication of risk for a single device.