Yeah, I do have to give him the nod that he did say worn out cassette. I too would replace a worn out cassette. But in my experience cassettes and chains don't wear out at the same time. Especially on the lower tier group sets.

I can't see it as foolish to only replace the chain if that is all the is ready for replacement. There might be high mileage cyclists that benefit from changing cassette and chain at the same time. But I think that rare. I've certainly never had to.

And I certainly wouldn't say it's standard practice. If it is, then how many miles a year must you ride to make this worth it?