[Nyororin]

I can sort of see where transcripts of television programming or anime may help an advanced learner. Either the stage where most things are understood but the new word or pattern thrown in every once in a while is a potential learning opportunity... Or someone who has studied written Japanese intensely, but has trouble keeping up with the speed of normal conversation.

But manga?
Manga is it`s own transcript, if it can even be called that. Transcripts are of spoken sources - manga is already in text form.