As for why "the rules" say that cell phones must be turned off, consider the position of the bureaucrat who has to make that decision. He (or she) can either decide that he will allow phones to be turned on in flight....If he does that, if there is any accident that is attributed to cell phone interference, even if it was caused by bad design decisions by the cell phone designers or the avionics designers on the plane, then he will catch the blame. Career ruined. And in fairness, he will no doubt feel awful about it. And there's no particular upside if he does decide to allow the cell phone use. He doesn't get a raise, or even praise. On the other hand, if he never allows cell phone use, then there is no problem, no downside. And he will never be called upon to explain a plane crash or accident. Nobody really cares about it, despite the grumbling.
If I was in his shoes, I know which way I'd go, and it would take me about .03 microseconds to make my decision!
As a person who flies several times a year, I hope and pray that cell phone use during flight is _NEVER_ allowed. I don't want to sit in between a salesman and a 16 year old girl, both of whom are talking nonstop for the four hours (or whatever it is) we're in the air. The chatter on a full flight would be ear-shattering, and make it impossible to read or watch a video, even with headphones. Since this would make it difficult for the pilots to get people's attention for inflight announcements, and even somewhat intrude on the attendants' ability to conduct the beverage service, I've often wondered if the airlines haven't quietly and informally asked the FAA to continue the ban.
Finally, I doubt the cell companies would be crazy about the problems created by dozens of cell phones zipping around at 400 mph and being handed off between towers every three minutes (or whatever the time in range of a tower would be, I made up the three minutes number).