Not many SMIL editors. Homestyle, Coldfusion studio have SMIL support. Believes there is no SMIL editor as comprehensive as HTML has for example. What do you do when your editor gets SMIL linking to a CC file. Also no tool to do general purpose editing of these files.
Captions depend on content.
We don't do authoring. Other apps do that. Integration means that you will take on all the burden of accessibility. If tracks are separate they can pull them. Hearing that people don't want captions embedded in video.