Apparently I hold these in higher esteem than most collectors. Like the M Frame, there are a ton of inventions and a lot of brave thinking that went into these. They certainly are on my top 10 list.
Jumping back to this, I don't think that they aren't held in high esteem. I just think if someone was after a lot of the early vintage pieces, then anyone who may have something to trade, most likley has a pair. Glasses are always on the top of the totem pole, so pristine Mumbos, Zeros, X-Metals, tend to not have equivalent personal values in other catagories. Also shoes take up a ton of real estate in the display cabinets, so it's hard to make everything work.