Leather breaks easier in an emergency (horse getting caught on something)
I also have found leather easier to keep clean, just give it a wipe over when you get the horse in.
I would never use anything other than leather but that is just me.The reason I do is the fact that leather breaks better when under pressure therefore less harm done to the horse.I have heard too many horror stories about nylon not breaking and causing serious injury.
Leather is more expensive, and it may look smart, but to be honest, who leaves their horse in a situation where it could break it's neck anyway - if you need to leave your horses' headcollar on the field or whatever, then you need to make sure it is on securely and safely.
I kinda agree with you, if you need a headcollar that will break buy a safety one with velcro inserts. My Arab would not be able to burst a leather halter anymore than she could a nylon one. Leather is safer for big horses if they panic, little ones will not be able to break the double stitching!