All of us can use common sense as well. When you evaluate a school and you see a GMAT average of 600 and a GMAT average of 705.64 which school do you think is better, and which one would you rather be a part of (it may turn out that all of the students at the 600 school are "better" but you will never get to know it).
Maintaining a high average has to be of interest in the admissions department. It is one of the most quantitative ways of insuring your "competitiveness"
however, this does not mean that they will not actively seek out interesting students who could not break out of that 700 score. It just means that they cannot disrupt the average.
Also, while it is true that some people might be very academic, but score poorly on the GMAT, it is probably true that very few poor students score high on the GMAT. Saying that the GMAT is a good way at finding students who can handle academic rigor is a no-brainer. However, finding the students who are exceptional who score poorly on the GMAT takes a LOT more effort.
So I guess the advice is...do well on your GMAT