You learn something each day, sometimes it has a big impact.
Suppose you are designing a FLV video player. Unless the player is to be extremely simple, you will want to support displaying closed-captions/subtitles, either because you are truly accessibility conscious and care about it, or, because you find the idea of displaying subtitles for a foreign language movie cool. Surely, you will want to cover all bases.
Lets start adding captions support to the design:
- You will need to have a button, labeled 'CC', that toggles display of captions.
- Not all FLVs will have associated captions, so a feature can be to hide the CC button (rather than displaying it in disabled state) when there are no captions available. This will also provide more space for other controls, a slider may benefit from the extra space. 
- Some people will want to start with captions-on, so you will provide the programmatic interface to start with the CC button already clicked once.
Can you think of any other option? (Other than supporting multiple language tracks) Anything missing?
Until some minutes ago, I wouldn't know the missing option, and the CC button would be the symbol of my accessibility support.
Missing option is having captions on all the time without the CC button to turn them off.
What good is that for? Obviously you could have thought about this, but why would you want it?
Because, at a site for the deaf (which is what accessibility is really all about in this case), a CC button is not considered deaf-friendly. Captions must be present and displayed at all times .
Just like the way you (and I) think, 'let's not display the CC button because there won't be captions for some FLVs', you should also think, 'let's not display the CC button because there are times there will be captions all the time and they will never be turned off'.
A FLV player with a CC button, sure, is accessible. But if you really care about accessibility, you should have the option for not displaying that button.
That's what I learned today, from one of our Captionate  customers, who is creating web sites targeting deaf consumers, and who himself is also deaf. He is passionate about this. For him, it's not an option to have the CC button displayed, it's inappropriate, it's not deaf-friendly, end of story.
Could I ever, by myself, have thought about supporting captions without a CC button? Maybe… Could I ever have shared the passion about this? I don't think so.
Nothing is as simple as it looks, accessibility included.
 Obviously, if you have designed a player that works with different skins, you will want to have skins without a CC button (no captions support) and skins with a CC button that support captions. For the sake of this discussion, this is irrelevant.
 'They can embed captions onto video in that case' will be too shallow thinking,
 We do not provide a player for deploying captioned FLVs with Captionate. We do not have a FLV player product.