It has nothing to do with the content of the exam (6th grade math or whatever) and everything to do with how candidates compare to one another in a more or less standardized setting.
The relevance of the GMAT score will vary with the reputation of the business school. Most of the top consulting firms (and banks etc.) would be happy to hire almost anyone from Harvard or Stanford, so GMAT score has little relevance in these cases. At a school like Darden where I am now, a high GMAT score can definitely give you a leg up in recruiting for competitive industries. It's easier to get into Darden than Harvard or Stanford, and school reputation alone is not enough to land the most competitive jobs, so other factors come into play. At schools ranked outside the elites (top 15 or so), a very high GMAT is one of the only ways a student can make it onto the radar of the most competitive firms; other include stellar relevant work experience or a blue chip undergrad degree.
So, for IB recruiting here at Darden, I think the following things are relevant: 1) prior IB experience (better if it's name brand), 2) blue chip undergrad with high GPA, 3) high GMAT, 4) strong schmoozing and social skills and 5) other interesting and relevant experience. You need to be strong in 2 or 3 of these categories to stay competitive. Say, you worked as an IB analyst and you're really great at schmoozing - great; you can be competitive even if you're GMAT isn't great and you went to a no-name college. Obviously, recruiters value things differently - some love high GMATs, others like strong social skills and others love prior experience.
If you have great prior experience, an outstanding college record, attend an ultra-elite school and have great networking skills, you don't need to worry about your GMAT for recruiting (it won't hurt of course). On the other hand, if you're a career switcher with an average GPA from a no-name college and average schmoozing skills, a high GMAT could really help. Each person can take a look at their own background and skills (be honest), the relative selectivity of the jobs they are looking at and the reputation of their business school and figure out how important a GMAT score will be on their resume. I don't think of it as a limiting factor, but rather an opportunity to make an impact - your background isn't going to change regardless of your GMAT score but a high score could improve the sheen on your resume.