Arm rests aren't obstructions. They are no more of an obstructions than the very seats themselves. For the aisle seats, where the armrests move, there is obviously the chance that someone could fall into the aisle. I don't mean falling arse over tit but certainly the upper body can get flung into the aisle. I guess the middle armrests are there merely for extra support and separation between passengers at a time when the aircraft is more prone to turbulence or sudden movement.