Different people have different standards of how they want things to work. Some also have different awarenesses. The first example that comes to mind.. I had a friend, a racer, come in with his bike for some shifting issue.. I take the bike for a test ride... and the headset is so worn out I can barely ride the bike... it is locked straight ahead.. it was the worst case of "indexed steering" I had ever seen. He was not even aware of it.
I have some customers that are not at all handy... and do not want to come back to the shop after their "spring tune up" so if I do not think their tire will make the season.. they want it replaced now[their idea not mine]. While others... will ride it until it is sufficiently worn out... some of course go way beyond that.
I myself can feel very subtle differences..and I want my stuff to work. to me a worn chain feels like crap.
yet I do not "push" my standards onto my customers.. We are a small enough shop so that we get to develope a relationship with most of them... and get to know what they want.. and generally the mech gets to personally talk with the customer of the bike he is working on.
To me ideally you do not mix chains and cass. of differing wear. Let's pick a weird example.. lets say you have a bike with a few thousand miles on it...and someone steals the chain. [yeah I know weird scenario] . The cass. is still good but has some wear. In this case I am of course "OK" with just putting a new chain on it. but this new chain will wear out quicker.. because it is on a used cass.
chains and cass. wear together.
I, as most people here use a chain until it is gone.. and replace the chain and the cass at the same time. when this is done on time.... chain rings last a VERY long time.
One of my co-workers.. will take a cass. use chain #1 for a certain amount of miles.. then put on chain#2 then chain #3.. then go back to #1 again. all with the same cass. It does works really well, but it seems like a lot of work to me.
He also has one main road bike... and I have many.. using his method would be a chore for me.
To me if a chain jumps or slips or comes off.. [because of wear] it is well beyond it's useful life. In this condition.. chain breakage is not out of the question... imagine hammering a hill and having a chain snap.
With what most of us are paying for tandems the price of new chains and cassettes are relatively cheap.
I do not have different sized cass.. for the same bike. {I may start that this year} If I did I would keep a seperate chain with each cass.. so that each had the same wear on them.
glenn