Yeah imagine having a guild and a whole city rocking up on your doorstep as a floating city travels past... Commerce, trade, etc ... I kind of like the idea they are not under control of the city upon its back, they are at the mercy of where the floater chooses to go, forcing them to be nomadic in a sense and still having to use the environment for scavenging. On the other hand the fluids (water) and flesh of the floater could make for a cool sustainability model. Or maybe the mites (being the size of an animal) could be farmed and kept under control, like whales and lamprey fish .... Thus the floater actually wanting the race living upon its back in a semiotic relationship... So many cool possibilities.