Perhaps splitting hairs (but this is the Playground, do we expect anything but the highest levels of pedantry? ), but I'm not refuting the idea per se - I think it's fine in the general case, but it happens to be of questionable value to my absurd example, or (as I explained) to the example from the OP. I agree that a single example of playing through backstory does not give me experience with the wide range of outcomes possible from playing through backstory; however, it does give me the portion of the experience specifically relevant to using a meatgrinder to generate backstory.
Or, you know, that's my claim.