> Back to marginal costs - the marginal cost of converting empty RAM to cache is zero in the minute, but only because the full cost has been borne up front when the machine is purchased. It's not zero, just pre-paid. .
Or, in other words, you get to fully use what you paid for.
I think the OPs point was that people tend to buy more RAM than they actually need because they have no idea how much RAM they actually need, because it's always used, and so err on the side of caution
Or, in other words, you get to fully use what you paid for.