Hello,
This is an old topic and I know that currently there are few issues with this on some old browsers.
Since there are more benefits instead of issues it will be more easier to switch it to always on demand if the subtitle is not working on a browser instead of change it to automatic. This happens when the browser gets updated, cookie expires, new pc, new location, new device, etc…
On the other hand it will be much interesting to be able to control this from the server side. Just a drop down box to be able to change the default value for all users. If an user is encountering issues he will be able to change this from his end. Also a check box for providing this ability to end user will be nice 
Thanks,
Alex
