actually, the whole question may become moot once Shimano comes out with their new electronic derailleur featuring a separate chain tensioner and derailling mechanisms. when that happens we can just about run our cogs and chains until the teeth on the cogs and the rollers on the chains wear completely through.

the tensioner, i expect, will mount on the chainstay, somewhere between the rear dropout and the BB shell. and it's single jockey wheel (with the chain running either above or below it) and sliding adjustment mechanism/motor will be triggered by shift changes. the necessary tooth count of origin and destination cogs are in a chip in the cassette.

and here's the important part. it will keep the chain rigidly fixed and with the proper tension at all times, with just enough slack to accommodate cog and chainring roundness and spindle and freehub concentricity errors. the advantage is that with a ridged and accurate tension the chain cannot skip unless the teeth on the chainring or cogs, or links in the chain are broken.

don't expect this anytime soon. because once the board finds out that they are paying employees in the engineering department to do something (i.e. prevent premature wear of chains and cassettes) that is at cross purposes with those they are paying in the sales department to do (i.e. sell MORE chains and cassettes), they will put and end to it, pronto.