Ahem, PoppaGator only said "minor" tunes; his scope was not expressly limited to the subset you mention and his postings to other threads suggest otherwise.

Even in traditional Irish and Scottish "modal" tunes, accidentals are not infrequent, and can be problematic on a Richter diatonic, Paddy or otherwise. A number of traditional tunes vacillate between modes while maintaining the same tonic. Dance tunes composed in more recent times introduce even more chromatic variations. Similarly, fiddle tunes not infrequently transpose for a B part (particularly on this side of the pond)--having to switch harmonicas at the transition points isn't a satisfying option.

In the middle octave, the Richters are incapable of producing either the augmented fourth or the minor seventh without overblows, and you need one or the other to move into a neighboring key signature on the wheel of fifths. You can only avoid this limitation by using a gapped scale or restricting your playing range. I fail to see how a less key-restrictive tuning "screws up your ability to play sets of tunes with key-changes" when in fact it does the reverse. It's only the range shift which might be inappropriate, but equally well, it can make a part of the range available which the Richter tuning precluded.

While Richters may accommodate your narrow purview, for the wider world they have limitations which can be avoided by using other tunings. In fact, in rebutting my first point, you suggested the use of yet another alternate tuning, one which isn't even available off the shelf from the major suppliers. It may not be difficult to retune a harp, but most folks would rather not have to do it.