Using cellphone while on flight is still forbidden by most air lines because of very good safety reasons. First, there are gadgets that pilots uses which are very sensitive and may be interfered with by the cell phone. This can endanger the lives of hundreds of passengers on board. It has been argued that passengers are the main reason behind the ban of phone usage while on board , more often before passengers board a plane you will hear them tell their friends that they are about to board a plane and therefore they may not be in a position to communicate while on board.
Since a plane may carry over 100 people, if all the people on board were to use their phones at the same time, this could be awful especially where businessmen were to talk and discuss for a number of hours discussing their business deals and ideas. Things would be worse if the plane was full of teens who would engage in gossips over their phone this would disturb the peace and a cool serenity enjoyed by the neighbors.
Technological growth may make things better by allowing more sophisticated airplane equipments that are not affected by phones, once this level is reached , it may be possible to allow phone usage while on board.
According to tests that have been done, it has been established that cell phones can greatly interfere with the navigation system of the plane. It has further been established that the radio beam which come from the ground cannot be detected by VOR making it difficult for the pilot to keep in touch with the people on the ground. However since VOR is quickly becoming outdated and is fast being replaced with GPS, things are expected to get better with time.