GPA is difficult, because there's no uniform standard. A 3.9 GPA from Caltech is much more impressive than a 3.9 GPA from Harvard (where there's serious grade inflation), but then again, you have to look carefully at which specific courses went into that GPA. But how do you compare those numbers to a grade of 15/20 from a French university, or a 1.1 from a German university, or some other country whose system you don't know?
Undergrad research (including a published first-author paper) is almost a requirement now for entry into many top programs. But you have to know the research group the student worked with, and you get applications from all over the world.
The subject GRE is a uniform quantitative baseline. It isn't everything (or even most of everything), but it is a somewhat objective standard that can at least flag applicants who might have real difficulty in a top program.
The thing is, I've heard "equity" arguments for disregarding not only the subject GRE, but also published research ("Not everyone has the opportunity to do research as an undergrad") and letters of reference ("Letter writers suffer from implicit bias"). The "equity" advocates seem to want to throw out every piece of relevant information. Of course, that makes it easier to hit whatever targets "equity" requires.