The problem with labourers who work in these secondary markets however, is the same as the guards who watch the gate: they can extract large tolls for being in the right place at the right time.
People in finance are rich because they're well-placed to skim highly productive traffic. However, it is -- in the vast majority of cases -- only skimming. The system functions very well to take unproductive surplus and allocate it effectively.
Though admittedly today, the larger beneficiaries are increasingly monpolies, and so on. But this isnt a side effect of the finance industry, but of the state.
How do you quantify this?
For markets to exist, monopolies must be avoided. As you can't expect large companies to police themselves, this was generally the role of the state. The states must strike a balance between keeping large companies happy (that want monopolies and have cash today) and true markets (which are efficient on the long run).
The finance industry is probably just a side effect of everybody focusing on short term (both public traded companies and politicians/states)
What did you measure to come to this conclusion?
Availability of free capital is a function of both just general wealth of a society, and how well lubricated the wheels of finance are.
I don't think this is a controversial theory, even if it comes with unpleasant side effects (white collar crime, inequality etc). Just as having a buoyant defence industry that can churn out a lot of boom is great for a country's war fighting potential.
Actual efficiency would dictate that there be only a relative handful of finance jobs, let alone very well-paid finance jobs. And that the vast majority of the money go to actually productive industries. And that the financial markets understand the principles and timescales of other industries, so they don't screw everything up with decisions equivalent to "Fiscal quarter ends in June, and Farmer Jones says he can harvest zero corn by then. Shut him down."
Whether they do so in return for services rendered or are extracting rents by acting as gatekeepers is a question that never quite gets resolved. A little of each I suspect, depending on the context.
If you want to call them a protection racket akin to the mafia... you're probably right.
Of course, banks these days are much more than that and there's plenty to rightfully crucify them for. But even still, there's a reason being called a third-rate bank clerk is an insult among the highest order.