I must say, Kris, that while the Brahms 51 address many issues found in Brahms, these issues are not idiomatic solely to Brahms. It is true that polyrhythms, for example, are frequently found in Brahms, but these exercises function secondly as studies in control of the thumb.
Some problems we deal with in these exercises include diatonic thirds, varied articulations simultaneously within the hand, thumb under, shifts, exploration of the contour of the keys, close proximity chromatic motion, rotation, wide reaches (cf. La Campanella intro), independence of fingers, trills, strength of pinky finger, and unison scales. These are all problems that we encounter frequently in the piano literature (some more than others). Certainly, the finger independence gleaned from these exercises is useful in more than a few Brahms pieces. But indeed, it is a requisite of Bach, late Beethoven (cf. the dreaded Hammerklavier fugue), and of course any other polyphonic repertoire that we play.
One can find in these exercises a few solid foundation in pivotation, rotation, and shifting, which are the three essential movements to piano playing. While Brahms may be famous for certain difficulties, they are far from being his alone. In fact, the graduate of the Brahms 51 has a versatile technique ready to face most of the world of classical music. And of course, as he has conquered the awkward (in order to play Beethoven, Bach, Brahms), it is true that he will additionally be at ease with composers who fit better under the fingers (Liszt, Chopin, Rachmaninov).