Each of my classes has upwards of 100 students. On every exam there's at least a page of some sort of free-response questions and problems (though there is also a multiple choice section.) I have TAs that help with some of the grading and I'm very grateful for them, but I usually grade most of the free-response portions myself, and it takes a lot of time. All told, I grade about 1600 exams/semester. I used to write explanations in some detail, but now I don't--and I do use the "???" because it at least keeps me from writing, "WTF?!"
Many of my students don't even bother to check the posted key, don't read comments, don't respond to pleas to "come in and see me about this topic--it is critically important and too involved to explain here." The few who care will come in and talk to me and I'll be able to help them. The ones who don't come in, I can only assume, don't care. So, the student who wants more than "???" on the paper needs to face a huge stack of papers on which the same comment will be written 40 times, will be actually read by two students, will be addressed MAYBE by one. Oh, and said student also needs to realize that the comment nearly always addresses something that has been amply discussed and emphasized in class. The instructor writing "???" on papers needs to take a deep breath when a student comes in to talk about it--that's exactly what we should be hoping for. Not that the student comes in to grub for points, but that there is an opportunity to look the kid in the eye and make the SAME DAMN POINT we've been making in class for eight weeks and hope that the eye contact makes it stick this time.
One strategy that I and some friends in other fields have used, with some success, is making a half-sheet, numbered list of common transgressions, and while grading, using those number codes to mark papers. "1" could be mechanical problems such as spelling or grammar, "2" math errors, etc. At some point on the list are "Complete misunderstanding of the point of the question," "Gross error in application of concept," "Irrelevant," etc. Students are either given the list in class (if the problems listed span the entire semester) or it's stapled to the paper as it's handed back (if the list is more specific to one assignment.)
One last thing: a grad school buddy of mine happened to be at a going-out-of-business sale at a hardware store, and picked up a rubber stamp with the word "FERTILIZER." I don't know if he ever had the nerve to use it on a student's paper, but I'm sure it must have been comforting knowing it was there.