I don't think that the bar will come off of your back...especially with a lot of band tension. If it does....freakin' wow. Remember that you can add several hundred pounds of tension to the bar just from going from the bottom of the squat to the top...it is a chore sometimes just to make it to the top..lol
The bands make the exercise feel more..."complete" I would say. They make the entire ROM count.
I use them on deads from time to time. Look at the Active Recovery threa on the Women's Board...it is a sticky. One of the pics shows Hannibal doing deads with them looped through his belt.
Never seen one snap...just make sure that they are anchored VERY WELL.