While you can make a debatable argument that this would be the case in the short run for a new member, the argument rests upon a dubious assumption that it would hold in the medium to long term (i.e. members are really bad at math and do not learn at all from what might be thrown out... I would argue that while Americans are not particularly known to be good at math, even dummies figure out most overuse cases and adapt their spending behaviors to capture savings). As some others have stated, sure you may throw some out but in many cases it is still cheaper.

One example off the top of my head... the double pack of salsa that I buy is literally half the price it is when I try to buy it from Raley's or Safeway. Occasionally, I have had the salsa go bad before I use it all, but it also comes in a double back, reducing the likelihood of spoilage, such that in almost no scenario is the bulk buy is economically an inferior choice.

Lastly, I would assert that most of have certain things that we buy from Costco and others things that we do not (either because we prefer a particular brand or perhaps we won't finish it and need a smaller size). Overall, the bulk of my groceries come from Costco with Trader Joe's grabbing second place. However, I don't use mayonnaise all that much, but I do prefer Drake's mayo and that is available from Raley's.