Ah! Another point - eyepiece pairs are not always perfectly matched to the nth degree as far as length and focus position. And they don't always seat perfectly the same way. The Denks allow the user to quickly and cleanly adjust this discrepancy out while viewing.
This was quite apparent to me recently when I finally got a chance to try out a pair of Leica ASPH zooms in my MkV - the adapter necessary to use the zooms caused just such a slight discrepancy. It was workable but still a bit annoying. Moot point in the end because I just didn't get on with the combo, just too large, heavy and unwieldy when all put together. The older I get, the simpler I like it (or maybe the simpler I get?).