Often when I’m explaining some of the details of on-page optimisation to clients, I tell them that many of the steps we take benefit not only search engines but also users using screen-readers.
Generally, the response we get is one of, “Yeah, but how many of our users screen-readers anyway? Surely it’s not worth considering something that accounts for so few users?”

I was reminded of this today, while reading this article: “A federal judge in California certified a class action lawsuit against Target Corp brought by plaintiffs claiming the discount retailer's website is inaccessible to the blind”

Win or lose, the case will undoubtedly draw some attention to the issue of Web accessibility and hopefully in the long run lead to better implementation of accessibility guidelines.
Even though the number of users specifically affected might be relatively small, the size of the effect for those people makes the effort worthwhile.