I don’t think the “average over multiple exchanges” thing really matters that much when we’re talking about the usefulness of market cap. The reason market cap isn’t a great measure of “total value at the moment” is because there certainly aren’t enough buyers willing and able to buy all Bitcoins or all Apple shares at the current market price.
True, but if someone tried to buy all the Bitcoins or Apples shares, the price would start to go up. Getting them all would cost more than the current market cap. This is why company buyout offers are always for more than the current market cap.
