Profit margins in the pharmaceutical industry are the highest[1] of all industries, only rivaled by the banking sector.
The top 3 pharma companies spend twice as much money on marketing as on R&D.
It looks as if revenues of one drug fuel investment into new drugs, and that may be true but it is misleading. There are a lot more sources of capital than just one particular companies own revenues.
Margins and marketing budget allow us to assume significant market power in favor of the pharma companies.
What is all that money being spent on?
Sales reps also help with a lot of the non-sales aspects as well (getting reimbursement for the doc, helping patients who can't afford the drug).
You are correct that if you have the best drug out there by far sales reps don't really drive that much in sales. However, it's tough to convince marketing that they don't need sales reps at all, so even for the best drug they hire them.
However they're exquisitely busy people, and no one is paying them to spend the time to keep up to date. Sure, there are programs that encourage it (usually called 'Continuing Medical Education') but they're often very weak, easily ignored and there is a lot of pushback by doctors themselves against them). If you take a look at the volume of information coming out every day, coupled with the fact most of the papers just plain suck, you'll realise how it's pretty much impossible to stay afloat.
In this hypothetical case, marketing $$ were spent to discover that existing late state melanoma treatments were lacking in some important respect (efficacy, side effects, cost...), and that there was a sufficient market to continue the expensive process of developing a potential treatment.
http://www.medicalprogresstoday.com/spotlight/spotlight_inda...
Haven't we had enough of high prices and stagnation in pharma already?
http://www.sourcewatch.org/index.php/Manhattan_Institute_for...
http://en.wikipedia.org/wiki/Manhattan_Institute_for_Policy_...