good example:
Doctor Who - world wide wanna-be
all the Doctors - starting with William Hartnell - were made rid of such heavy accents that the rest of the non-UK English speaking world could understand them . . until...
Peter Capaldi - at first it worked, then his accent became thicker and thicker until a non-Scot could understand only about 50% of his dialogue. I actually stopped watching his episodes. he carried the character well, but was next to impossible to understand.
so sub-titles are very useful.... then again, , , (fluent in German) watched Das Boot in a German airport cinema 'between flights'... and then saw it later on US TV with sub-titles . . . sub-titles lose an _enormous_ amount of the original. the small aside comments, incomplete sub-titles of what was actually said, the background comments . . . do not appear in sub-titles, nor the inflections and verbal 'eye rolls'
so, they're a yes&no thing....