How much more useful are ever higher pixel densities?

True.  Any decent lens will at least be even with any known DSLR, center to mid if not the whole frame.  The D5200/3200 are slightly more of a challenge, but finer detail than any 16MP item is clearly visible using the kit lens or any of the cheap primes for that matter, such as the 50D or 351.8. Some would prefer that not to be the case, probably because they own one of the lower res bodies and are perfectly happy with same, but I guess that's just human nature.