The ban on cellphone usage during takeoff and landing is for your safety. The ban on cellphone usage during cruise is due to weaknesses in the cell network and your sanity.
The reason we tell you not to use your phone for takeoff and landing is because those are the point during the flight when the aircraft is most likely to encounter problems and also when our navaid usage and workload is at its highest. We are trained to assume that the airplane will crash on every flight and act accordingly - Complacency Kills! You should be in the same mindset. First, there's the matter of the crash. When the aircraft goes from flying speed to nothing in a few seconds, the G-forces are going to make that iphone/laptop/whatever that you are holding in your hands suddenly weigh several times its normal weight. You WILL NOT be able to keep ahold of it. It's going to become a projectile and injure or kill the people sitting near you. Next is longer-term survival. The fact is, most deaths in air crashes happen not during the impact sequence, but in the post-crash environment. People panic and stampede. They don't know which way is out. The aircraft is dark and possibly filling with smoke or water. Situational awareness and decision-making ability are KEY to both your survival and that of your fellow passengers. Having to get your headphones off or figure out where your laptop went is not going to help. If you weren't paying attention to things before the crash you won't know where you are now and what direction you need to go. You probably ignored the safety briefing too. See where this is going? Finally, if you are alert and paying attention, the amount of information you will be able to provide to the crash investigators after the crash will be of higher quality. Those of us at the pointy end of the aircraft probably died in the impact. Being able to give information to the investigators could uncover flaws in the aircraft or our procedures, and by correcting those save hundreds of lives. We take this flying stuff seriously. You should too.
I've heard that cell usage during cruise overloads the cell network by switching cells too often - I'm not an expert on the cell system so I'll defer to a cell tech on that. In my eyes, the ban on cell usage during cruise is for reasons of everyone's sanity. Do you really want to hear the guy in the next seat shouting into his phone about the BIG IMPORTANT EXECUTIVE THINGS BIG IMPORTANT EXECUTIVES LIKE HIM DO, or THIS THING ON MY NECK IS GETTING BIGGER, or whatever other inane thing he wants to rattle on about at maximum volume? It's bad enough everywhere else, why must we suffer too? (Misery loves company?)
Anyway, that's the score. I've repeated this I don't know how many times now and it never sticks. STICK, DAMMIT!