I've been through plenty of PTO shafts over the years, and ignorantly started out using the chains to stop the PVC cover from spinning. Didn't take long before the little PVC connection point gave up the ghost. Chain dangling, cover spinning. Just another example of incompetence at OSHA.
So the only thing I use the chains for now is to keep the shaft up outa the dirt when the implement is parked. The housings still spin, but I just don't let anybody near them that could get hurt.
//greg//