If you want to sell to the government, you've got to have accessibility provisions. Educational institutions and just about all companies with purchasing policies have disability requirements, too. Other aspects of MSAA deal with how accessible your program is to Java. Although there's no requirement for internal developers working on proprietary apps to comply, they'd be well advised to do so. Your in-house code should function at the same level as commercial packages.
Accessibility is hard work, but ultimately it's a win for everyone.