Eh, I used to think so too. And being from the Austrian school-of-thought I would agree with what Mises says, but when considering the productive output of the world, it is (at the very least) a measure of transactions. This can be handy when you have a government that provides a currency that gets inflated, but it becomes kind of useless when said government quits recording M3 so you can see its impact, so I digress.