It's hard to design, write, code, etc. for people you don't know. One of the value of usability testing with people with a variety of disabilities is that it puts a human face on the goals of Section 508.
I do NOT mean using people with disabilities as "accessibility QA testers." This is about observing - and understanding - why each requirement in Section 508 (or WCAG 2.0) exists: what human communication need it meets.