The issue isn't that interference can't be caused in cruise (it can), but that takeoff and landing are more critical times. PED interference is documented; it happens. I've experienced it once that I'm aware of, although like most cases of PED interference, I cannot without question state that it was a personal electronic device. The situation couldn't be duplicated, and I could find no other explaination.
If you review the ASRS data base, you'll find several hundred reports detailing control and navigational problems that have been attributed to PED interference. While it's logical to conclude that a certain number of those reports may be crews trying to pawn off a mistake on a catch-all problem, I believe that many such reports are indeed genuine.
While at first blush a small electronic device may not seem like much, one has to appreciate that it's being operated in an environment which can amplify or alter the signal or signal byproduct, and which places wire bundles and devices, antenna coax, and a variety of potential receptors. Any electronic device outputs electromagnetic fields and electromagnetic radiation in a range of frequencies. One of the harmonics for a generic CD player, for example, coincides with VHF nav signals, and can cause interference with VOR and ILS reception. The interior or the aircraft tends to reflect this signal, and there is always a potential for interference.
The topic is hotly debated, but that doesn't change the fact that numerous cases continue to be reported of PED interference with flight control and navigation. Most commonly navigation.
I'll restrict my comments here to generic terms. I'm not an electrical engineer, nor an avionics tech. I know that some folks who read this board are engineers and have avionics backgrounds. For them, I say please ignore my simplistic view and potential misuse of terminology. The sole point of my comments is that PED interference is a genuine concern, or should be.
The potential for interference enroute is the same as departure or arrival. However, the risks are potentially higher in the terminal environment, or close to the ground.