I have very little to add that RBark, Nicole, Romy, Sael and Fran have not already said.
I have an invisible disability. I don't even like telling my family what it is most of the time. I live with it for the most part, and I don't really go anywhere I can't take my dog anymore anyway. Enzo is not an SD. She washed out, and so she doesn't go anywhere that dogs are not allowed. When she was in training, before she washed, I didn't NEED my husband, or my mother to go to the book store or the grocery store with me. I didn't need either one of them to come find me in a store if something happened and I got disoriented.
The not needing other people to just function as a normal person, to go grocery shopping by myself, that was worth it. The stares, the questions, even the people who screamed if they came around the corner and saw the dog. (seriously, do you have any idea how humiliating that is?, not nearly as humiliating as having the store manager come over to you when you're "stuck" somewhere and ask you what the hell you think you're doing.)
But to require certification, how are you going to fund that? How are you going to get people who are on a very limited income (I seriously live on less than 20 dollars a week, after paying bills that are necessary). I would never be able to afford traveling to somewhere to pay for a test to give me rights THAT I ALREADY HAVE.