I can play a 10-hole diatonic G harmonica in G, A Dorian, E Aeolian, D Mixolydian and C Lydian. I can play a 10-hole D harp in D, E Dorian, B Aeolian, G Lydian and A Mixolydian. OK, so Lydian is pretty infrequent. I can, and do, switch harps in sets of tunes. I use an A harp quite frequently in addition to the above if I need A major. Apart from the A major in some cases, I believe you can do all those on a simple D/G melodeon. Not only that, if someone decides to do a set in Bb/F, you can always drop out for a bit. This discussion strikes me as being somewhat spurious.