Okay, so I have done about fifteen minutes worth of “research” with Google and as far as I can tell there is no existing justification for qualifying and/or preliminary exams (I mean, of course, aside from, “Well, that’s how we’ve always done it,” or, “I had to take quals, so these kids should take quals, otherwise they’re not real Ph.D.s”).
Here are my thoughts.
1) If you were accepted into the program and offered money and all that, presumably they think you’re good enough to succeed, and trust that the grades you received from your undergraduate institution are meaningful. If not, why did they accept you?
2) If you passed all your graduate coursework with the minimum or better grades required for your program, presumably your institution believes those grades mean something. If not, why do they care what grade you get in the classes?
3) Given the circumstances in 1) and 2), there is absolutely no logical reason for a department to administer either preliminary or qualifying examinations, as their existence essentially renders moot any previous assessments of your work.
Therefore, I declare that we must put a stop to this heinous practice immediately, as it is contributing to the declining value of letter grades everywhere! It is also contributing to the declining value of my Saturday mornings, which, depending on your perspective, might be considered even more heinous.
I think the University of Texas at Austin’s Department of Physics has got it right. Instead of quals, you take “core classes” (I think there are four) that cover the main topics in the field (Mechanics, E&M, Quantum, and probably Thermo or something like that) in addition to more elective-type courses. The classes are harder than usual and you have to get a particular grade in them. But if you pass them properly, then it is assumed that you both a) had sufficient preparation from your undergraduate studies and b) are sufficiently prepared to continue with your research and your career as a physicist, as per the standards of that university. Unlike other universities, these core classes are not separate from your standard coursework – thus eliminating the ridiculous redundancy that is your average qualifier.
Excuse me now, I have to go put away this soapbox.