Webster defines "weed" as: "a plant that is not valued where it is growing and is usually of vigorous growth; especially : one that tends to overgrow or choke out more desirable plants"aren't dandelions considered "weeds" or "pest plants"? so wouldn't having a dandelion farm be considered as a nuisance?
So basically, classifying something as a weed lies in the eye of the beholder. If one would have a dandelion farm, dandelions would be both "valued" and "most desirable". Of course turning your front lawn into a dandelion farm might voilate some covenants and would be pre-defined as a weed by the local property association (a higher power).