Originally Posted by Mike Leary
They are great if you want service work: you not only add water into a drained main/lateral, putting stress on everything, and, since they're installed at a low point, crap can plug the drain, rendering it stuck open or closed.
I won't use them on laterals, but I will use them on mains or manifolds. If we have to repair a main/manifold due to freezing, quite often it is because either the customer did not drain it (or drained properly), not been blown out, or there is a low spot or incorrect level for draining. To prevent this from happening again, I will put one in the manifold or main line. Sure they can fail, and sure they can eventually get roots growing in, but even a brass manual drain fails. I have had more brass/cast drains fail and fall apart than auto drains. Or the rubber inside cracks and breaks away while it is operating.