Obviously I'm a bit biased given that I fall into the high MCAT/low GPA category but this line of thinking really bothers me because it fails to take into account the course of study and the undergrad institution. Isn't this part of the reason the MCAT exists in the first place: to provide some objective measure of a candidates suitability, since GPA alone is dependent on these factors. I've studied now in 4 different environments : high tier state research university, private research university, mid-low tier state university, and community college and can attest to the fact that different universities vary widely in academic rigor of the classes and in the grade distribution. Furthermore topics in different programs at the same school can vary significantly, physics for engineering and physicists at my undergrad institution was calculus based and vastly more difficult than the physics for bio/science majors which was algebra based (not to mention the impact of the curve in a class of all engineers and physicists). Even different sections of the same course at the same school often result in one section earning a reputation of being significantly more difficult.
Also RE: the idea that in med school everything is universally more difficult than undergrad:
SMP physiology : exact same course as med students (algebra based), tests exclusively multiple choice. 92+ was an "A" for SMP students, which I earned in both sections. The med students need just a 70 to pass. Med students petitioned to have difficult to understand double negative question stems removed( lol?)
UG BME physiology: pre-reqs: differential equations, multivariable calculus, two quarters of single variable calculus. Outside of this the actual information covered was about the same but because the examinations were open answer and required the use of multivariable calc/ solving ODE's it was definitely more difficult.
So before lumping all of us low-GPA'ers together as having issues with "substance abuse or psychiatric disorders" please consider that not all GPA's are equal to begin with. I felt like in my engineering program it was often a struggle just to pass certain classes even if you did everything you could. I studied my butt off for biomechanics only to freeze on one of the five multi-part questions on the final and end up with a D the first time around (covered this wonderful book
http://www.amazon.com/First-Course-Continuum-Mechanics-3rd/dp/0130615242 from cover to cover in only 10 weeks, check out the pdf which can be found in google, just looking at it is like a traumatic experience for me) I know several students who had to switch out to other majors because they could not pass certain courses even with multiple attempts. I know engineers with GPA's <3.0 who are now very successful in industry, sometimes with GPA just above 2.0 even. This definitely needs to be considered when making up the DDx.