Roger's observation on their composition is correct, but their allure is based on personal collecting preferences. Just as the decision to purchase a survivor car as opposed to owning a pristine frame off restoration or indeed even a resto-mod, can all represent extreme variations in condition and collector psychology. What motivates a person to collect can vary. Back in the day these models were made from a cellulose nitrate based plastic which had a tendency to deform with age and perhaps exposure to heat and light.
In the collecting world some things are known to simply not survive well. Rarely however, does that mean that such objects are NOT pursued by collectors. I would never have collected early pulp paperbacks if I had objected to browned, brittle acidified paper, but that is the way those particular objects have survived due to the material they were made of, for example. Sometimes defects ARE the allure to the collector, & sub-sets within the collecting world that recognize such develop. Good examples of this are found within the numismatics field. It comes down to why one chooses to collect. These are "real skookum" as the expression says, and they represent an odd, period GM licensed model of our cars, which in itself is very interesting to some. Clay/Lexi