They are what most collectors want for their collections. A repainted plate is generally not worth any more than a rusty but original version of the same plate. However if a certain plate is particularly hard to find in good condition, a professionally restored version might be desired by collectors. Reproduction or counterfeit plates are to be avoided. Many widely counterfeited plates are easily found in their original form through other collectors.
Many collectors collect prop plates.
The oldest plates, pre-1920, are among the most prized plates. Even rusty or repainted pre-20 plates from some states are a rare find. The most valuable plates of all are pre-states, usually made of leather or porcelain, that were used before the states directly issued plates. From the 20's through the 50's, plate values tend to diminish as one might expect. These were either small metal tabs to revalidate last year's plate, or full-size plates made out of fiberboard, soybean, or some other material.