If you look at statistics, the number of families with children with a mild special need has increased dramatically. I do not think a mild special need warrants skipping the lines. Otherwise the majority of families at Disney would have a GAC making the GAC pointless.
In my opinion, Disney should require proof. People talk about the legalities, however, no one is forcing the family to ask for a GAC. I am sure there can be legalities that if you want to voluntarily ask for a special privilege, then you should voluntary disclose the proper information or provide a note from a medical provider. A note from a medical provider does not have to go into detail over the special needs, only annotate the general diagnosis or condition warranting the accommodation. It's easy. If the family doesn't want to disclose, then they don't have to and Disney in turn does not have to provide a GAC.
This just shows Disney does not want to upset guests by asking them to validate their claim.