Here’s a parallel concept: Not everyone who practices usability is an expert in information architecture, but I think everyone would agree that the fundamental principles of information architecture are essential knowledge for usability professionals.

An important role of the information architect, then, is to develop new strategies for getting IA done right. Another important role is to educate the rest of us about the most useful of those strategies.

Similarly, it’s important for everyone who works on the Web to know about accessibility and do their best to make their content and its features accessible. But most people can’t review their design from the standpoint of all the different scenarios that can arise with respect to accessibility. That’s one place where an accessibility expert can help.

Accessibility experts can also help people see the value of their efforts to make a document more accessible. Take your example of synchronized captioning. Most employees where I work are not provided computers with speakers. Some don’t even have audio cards. How are they supposed to follow the sound track of a training video? Synchronized captioning would solve that.

Admittedly, that lack of hardware is becoming less of a problem as more older computers are being replaced. But what about someone who is constantly on the phone and works in a cube? Should they have to go back and forth from headphone to headset as they follow a video between calls? Synchronized captioning would remove that problem.

Just as with curb cuts for sidewalks, improvements to the accessibility of electronic information ease many predicaments beyond those of the targeted group. Helping the average practitioner see those benefits is an important part of the accessibility expert’s role.

Think of the accessibility expert as mentor, guide, and coordinator. Others still need to do the work, but the expert shows them the way.