There are a number of issues associated with viewing disability as a binary as opposed to a continuum. We will look at the Autism Spectrum as an example to outline these issues.
First of all, exclusivity becomes a problem when you are treating it as a binary. When you do not treat disability as a spectrum, you begin to exclude those who are not at a certain level of disability. With autism, for example, there is a vast number of people with Asperger's and similar disorders that are very high-functioning members of society but who do need certain special accommodations for their disabilities. These people are excluded if you treat it as a binary.
Secondly, there is an issue with devaluing people. When you treat disability as a binary, there are those who are "able" and those who are "disabled" with no in-between. Dividing individuals into these categories automatically labels and devalues them. By saying that someone is unlike all the "ordinary, able people" makes them seem less valuable and less worthy as a human being, and this is a very slippery slope, especially with ethics and legislation.
Finally, an issue of treatment and study arises. When you are examining people with disabilities such as autism, if you are only accepting the extreme cases as people who genuinely have the disability, then your study and treatment practices will be skewed. It may well be that subtler changes are at work in their genetics or mind (for instance, with depression or a similar mental issue) and that studying the most severe cases will result in scientists and professionals overlooking the subtle influences that impact the disability. In many cases, this sort of situation occurs, and professionals will treat the symptoms of a disease when they mistakenly believe they have found the cause of it. The true cause tends to be harder to assess when there is a myriad of debilitating symptoms that need to be addressed concurrently.
Viewing disability as a binary condition rather than placing it within a continuum is problematic for numerous reasons. One of the key problems that such a view raises is the promotion of ableism, or discrimination against people with disabilities. Ableism is furthered through the closely related problem that a binary contrast places “disabled” in strict opposition to the supposedly “normal” condition of ableness; this opposition makes people with disabilities seem deviant or abnormal. A third problem is that grouping all people with disabilities into a single category obscures the wide range of conditions and behaviors that can be considered disabilities.
Ableism is increasingly identified as a significant obstacle for people with disabilities. Numerous prejudices and types of discrimination against people with disabilities include stereotypes about the relationships between physical and mental conditions. Such discrimination often supports the continuation of policies and practices that disadvantage people with disabilities.
Positing disability as a binary also promotes the idea that there is a single standard of normality, against which people with disabilities should be measured. In such a scenario, only a tiny fraction of human beings would fit into the able category. Promoting a binary opposition also discourages the acknowledgment of the wide range of conditions that have been or may be considered disabilities.