I agree that for many many jobs, hiring the hardest working most dedicated person is more important than hiring the brightest, most intelligent person. On the other hand, the most competitive jobs (MC and IB) require that candidates have it all.
I've spent substantial time working across from IB folks, and I can say for certain that it's not possible to succeed by working harder. These people work 80, 90, 100 hours a week regularly. To get to this point, they were already the brightest and most efficient.
Remember, we're really just talking about maybe 6-8 management consulting firms and maybe a dozen investment banks. This is just my opinion, but I believe that these firms believe that high GMAT scores will come pretty easily for those that are qualified to work for them. Firms don't care about what someone learns when the study for the GMAT; to them the GMAT is a known quantity that candidates can prepare for.
All MBA candidates are faced with the same problem known as the GMAT. People that are able to quickly adapt and learn the skills necessary to ace the GMAT also be the people that are able to think quickly and adapt to challenging situations on the job. I think you are missing the point if you believe that firms look at the GMAT because they are interested in how good you are at DS or RC or whatever. They want to know that when faced with a challenge, you can rise to the top of the heap.
Regarding whether an MBA is necessary, most of the top firms do recruit a lot of people with advanced degrees other than MBAs. When I worked in NY I knew many lawyers that moved over to become investment bankers. McKinsey and company is known to recruit actively at top law schools, PhD programs and even medical schools. Of course, regardless of the school they are recruiting at, they will want to look at your test scores (I know this to be 100% true for the LSAT).
For someone not looking at MC or IB, then the GMAT is much less important. As mentioned above, recruiters will assume that you scored below 700 if you don't include it on your resume (during MBA recruiting of course). Now, this may not matter at many companies - I gave the example of my friend where he was sure his scores didn't matter. But there are other competitive industries where GMAT might still factor in.
Whether a high GMAT is a good indicator of success is another question altogether, but firms are reasonable entities when it comes to hiring. They wouldn't rely on data unless they believed it correlated with successful hiring. They don't ask you to indicate your shoe size or hair style in their application, but MC and IB firms do require that you indicate your GMAT. You can draw your own inferences.