I'm sure you guys already know this, but the chain is really "worn" and not so much "stretched".
I know when I work/pull my bicycle chains apart (quite regularly, I cook them in wax for lubrication) there is a slight amount of wear on each pin/link. It isn't much, but once you add it all up over the length of the chain...
From what I can recall the drive sprocket on most chainsaws looks nothing at all like a bicycle sprocket, nor does the chain. They are assembled in the same way though. I suspect it may be a bit more forgiving than a "normal" sprocket as the chain wears.
I know on my bicycle a slightly worn chain is easier to lube than a new tight one and seems to stay lubed longer.
Personally I use chain saw chains till the teeth can no longer be properly sharpened. Only use a file too, I can't see the purpose of a grinder that removes way-to-much at a time. The Dealers like grinders, lets them sell a whole bunch more chains :)