i tend to think that it is carving out space with every turn of the crank... more so when dirty.

in my mind the chain is alternately applying varying amounts of force, depending on how hard one is pushing on the pedals, on the back edge of several teeth on the cog at all times, wearing that back edge. unfortunately, as it wears it also ramps up,

edit:



(two cogs, worn and unworn, superimposed, eventually the chain slips over the top given enough force and chain slack, and the feeble spring in a derailleur's chain tensioner can accommodate that with ease.)

end edit:

and the chain begins to ride higher and higher on the cog. eventually it begins to slip over top of the cogs. this is facilitated somewhat on bikes equipped with derailleurs and their spring loaded chain tensioners (the phenonenon is rare on bikes that lack them, in as much as it is difficult or next to impossible for the chain to skip over the teeth due to a lack of slack available in a properly tensioned chain), this is observable on a well worn cog/chain. in my mind changing to a new chain cannot stop the process.

like i said, it's not much, cost wise, IME, unless one is buying titanium cassettes, and if i did buy titanium cassettes i would probably think about the whole issue a while heck of a lot more than i do now. and want some serious independent studies that provide convincing evidence one way or the other WRT to longevity. i know of none, but i would find the reading of interest if i were to find any.

it's really just another of those "lube" issues.