It’s not really the GPA itself that’s silly, it’s the idea of using GPAs interchangeably, without looking at the transcript. Yes, engineers do tend to have lower GPAs than students in some other areas. At my university, this would be somewhat masked somewhat by the fact that a relatively high GPA is required to be admitted to an engineering major, and to stay in it. Before admission to the major, the course work has to include a lot of calculus, calculus-based physics, and chemistry. Separate overall GPAs and STEM GPAs are computed, and to get in, the student has to cross the threshold in both. This means that the students majoring in engineering have–on the whole–stronger academics than those majoring in many other areas. I can’t comment on Econ specifically, but I would suspect that students in math-intensive subjects also tend to have lower GPAs than those in less demanding majors. Of course, it is possible that math majors have very high GPAs, if only those who are very gifted in math wind up majoring in it.
My university tracks the average grade awarded by class, by semester, and by college (which boils down to area of major–engineering is a separate college, as in business, science is separated from arts and letters, etc.). We also track the difference between the average grade assigned in each course, and the overall GPA of the students in that class. This is quite informative. But we don’t issue the results to the students.
Some colleges (e.g. Columbia) list on the transcript the % A grades awarded in a class, as well as the student’s grade. The %'s tend to vary a lot by course. Some colleges (e.g. Cornell) list the median grade in the class. This is also illuminating–though a grade of A+ in an advanced mathematics course loses a just bit of its luster when one learns that the median grade in the class was A+ (but it could be that only very talented mathematicians got that far).
I think the MCAT has some ability to show what students have learned and what skills they have developed. The GRE does that less well, if one tries to compare students from different colleges. The general section of the GRE is not greatly different from the SAT. In physics and mathematics, the subject tests tend to emphasize the first two years of university course work, so quick response based on an excellent mastery of that material is very helpful, but it doesn’t get too far into the more complicated work. Also, in my field, the interpretation of the subject GRE scores is somewhat skewed because of the large number of international students who take the test, often when they are close to completing a master’s degree, and sometimes when they have also served as a lecturer in the subject for multiple years. Then too, they tend to prepare very heavily. Near the high end (top scorers) in my field, the percentile corresponding to a given GRE subject test score has dropped over the last 30 years or so, as more international students have taken the GRE subject test–i.e., a score around 900, which used to be 98th or 99th percentile is now about 90th percentile. Not sure about scores of 950+. (Also, the high-end subject test scores might have been truncated at 900 or so more recently than the last time I looked at percentile conversions.)