25% of Americans suffer from a mental illness and only a fraction of us seek help. The ADA does not protect the mentally ill. That said, with the recent nightmares of mass shootings at the hands of either poorly treated or untreated mentally ill people we can no longer afford to continue to enable the stigmatizing of mental illness. Giving them legal protection insures that they, we, can manage our disability with the same level of respect and safety that is now assumed elsewhere. If we do nothing, then we are merely waiting for the next nightmare.
Please note, that while the ADA does "cover" mental illness, It does so in name only. Filing a complaint with the EEOC or with a company requires you to fill out forms that only target physical disabilities. That is especially true if you request accommodations since reasonable accommodations for a mental illness are going to be very different from such for a physical disability. Consequently, when you request accommodations or EEOC support, it is effectively impossible for the mentally ill to satisfy the legal and policy requirements. This failure enables the ongoing systemic bigotry that ultimately disenfranchises the mentally ill.