Stainless steel and nylon rings are regularly used to make indirect returns. There is little load on these parts and little grooming. Their main purpose is to keep end circuits clean and reduce shells. The advantage of this type of product over low-friction rings is essentially its price. If you can afford it, go for low-friction rings!

The advantage of rings over blocks when there's little load in the circuit is that any twists made during maneuvers pass through more easily, since the opening is larger.