Geofitz13,
Gearing a 20-inch drive wheel has some consequences. Most bent riders try to max out the gear *range* and in that respect 20-inchers are at a disadvantage. With a 26 or 27 inch/700C drive wheel, typical chainrings are 30/42/52 but with a 20-inch they are 39/52/62 to get in the same neighborhood for high gear. Front derailleurs work best with a 10-12 tooth difference between rings. Problem is, with the larger rings, that 10-12 tooth difference represents a smaller % of gear change. So the upshot is, 20" drive wheels suffer from reduced gear range. Whatever you do to reduce/eliminate the disadvantage can also be done to the larger wheels, which puts you right back where you started, namely at a disadvantage. Of course, it's only a problem if you're trying to push the envelope. Many people are happy with the gear range available. Other disadvantages of 20" would be higher rolling resistances and faster tire wear. I think there's exactly one model of tire available for ISO 406 rims that's narrower than 28mm but plenty of fat tires available.
I'd say that the advantages of 20" would be compact size, which manifests as a shorter bike and a lower seat, and in the case of dual-20, only having to worry about one size tire/tube. (The last is also true of dual-24, dual-26, etc.) Weight savings for 20", IMHO are insignificant due to the unavailability of skinny, lightweight tires in that size.