Great posts!! If you have ever really long-lined a horse, specially a young one, YOU will tire out long before you do any damage to it...... we are not talking about putting a chair in the arena, having a seat,and letting the horse run willy-nilly around. This is about teaching a horse(and driver) consistent contact with the bit(quite difficult from 30' away), moving laterally on the circle, and accepting whip and voice cues.
On a young horse, I run the lines through the stirrups, or tugs, or lower rings on syrcingles, so when they get confused and spin, the lines are low to make the correction. otherwise, they get twisted around.