I can't personally say for software, but in any other arena 'accessibility' generally makes for better products/services. Curb cuts, for example, serve those with mobility issues. We think people permanently in wheelchairs, which is true, but pretty much every human is going to have mobility issues at some point in their life - injured, pregnant, just carrying too much, dead drunk, etc. So in short the business value of accessibility is similar to the business value of quality.