Also interesting about a worn chain is that it doesn't affect shifting either. Theoretically a very, very worn chain (so worn that it will only engage on your most favourite sprocket and skip on all others) combined with an RD that's set with the top idler pulle far away from the sprockets, may not shift into gear, but that's not a likely scenario. Therefore I feel safe to say a worn chain does not affect shifting. Take a worn chain and test it in shifting. Compare it to a new chain. Big difference.
