FarHorizon

I'm not philosophically in either camp (replace or ignore) at the moment, but it seems to me that the SIDE WALLS of the derailleur cage are what become the effective shifting mechanism once the teeth are gone. Since there is more distance between the cage side plates, shifting should logically become sloppier once the teeth are gone. Additionally, there should be more friction as the chain rattles against the side plates than if the chain is being centered by the jockey wheels. Of course, all this assumes that the teeth are TOTALLY gone, and that the chain is free to move laterally within the cage plates of the derailleur.

So long as the jockey wheels are doing their job (keeping the chain centered and out-of-contact with the side plates), it would seem that the condition of the teeth would be academic. In other words, significant wear to the teeth themselves would make no difference in shifting so long as the side-to-side width of the jockey wheel is still sufficient to control the chain. Once the jockey wheels become narrowed by wear to the point where they no longer center the chain (even under the lateral load of shifting), then it would make sense to replace them.

Obviously, all the above paragraphs assume that the jockey wheel BEARINGS are still smooth and frictionless. If not, then energy is lost as the wheels are being turned (resulting in accelerated wear on the teeth) and the bearings should be maintenanced or replaced.

I may be off base with these musings - I'm typing as I think...