First of all we should clear terms: are we talking about accordion or harmonica style reed plates (i.e. single or multi reed plates) or are we talking about accordion or harmonica style reeds (i.e. thicker reeds possibly made of stainless steel or thinner reeds made of brass)?
This seems quite important to me because as far as I know the reed plates don’t influence the sound much. So what are single reed plates good for? Can you change them more quickly if a reed has been damaged? If they are screwed yes, but mostly they are glued with wax, so you can’t change them that simply. So what’s the point of waxing?
The technique comes from the accordion where you want to avoid wear and tear from screwing into the (wooden) reed block. The reed block is made like a series of small chambers. On the one end you have the valve, on the other end you have the reed plate which you glue with wax it to seal this side of the chamber completely. So maybe single reed plates for melodicas are just a residual of this accordion technique which is redundant for plastic or metal melodicas (just remember who uses or used single reed plates: accordion manufacturers like Hohner, Victoria, Ballone Burini and the manufacturers of accordinas).
So, the crucial point for me seems to be the reeds themselves as the base of the sound. Like Johan I prefer the accordion style reeds because I like a stable sound over a wide dynamic range. (And like Johan I have got the feeling that the sound depends as much on the reeds as on the body material and the sound channeling inside of the melodica. It would be very interesting to compare the sound of different reeds in one and the same instrument or even outside of a melodica!)
BTW, Daren, I’m not sure if your impression that the Vibrandoneon is harder to play than other melodicas comes from the reeds themselves. Those are highest quality reeds, and when you play them in an accordion it is a dramatic difference in response and dynamics in comparison with mid or even higher class reeds. I suspect that it is the bigger air chamber that makes it harder to play although on the other hand this may create a bigger, rounder sound.
A little story about “The emperor’s new clothes”: I went to a brass manufacturer yesterday to make him create a new tube for my Vibrandoneon. A friend of his was there, too, and he asked me to show the instrument to them. There was a great “Wow!” – and they didn’t mean the sound but the looks of the Vibrandoneon, they weren’t even interested in the sound.