How about this idea? - those spikes would have been big enough to sever or at least badly damage the major nerves in the arms, hence rendering said arms useless/paralysed so the victim couldn't have extracted themselves from their position. Would it be reasonable to think that the victim would have been more preoccupied with keeping breathing?
Another thought - I don't know about anyone else, maybe its because I have rotten circulation but if I had my arms raised in the position as shown in the BBC version, the blood quickly leaves them and they become incredibly 'heavy' and I lose any ability to control them, they just drop by themselves - if they were nailed or just tied in that position (so regardless of nerve damage) I would be powerless to make my arms do anything.
Also because they were crucified in 'squat' position with a 'seat' (can't remember the Latin term, sorry!) to stop them from completely hanging by their arms, the spikes wouldn't have ripped along the arm towards the hands.
I do think the soldiers would have had to tie their victims down in order to nail them though! And weren't the victims usually left on the cross for a few days too? If so, the soldiers could have removed the spikes (valuable and rare commodity) and left them tied with the ropes. So nailing and tying might be more of the norm.