Yes, the idea would be to have some kind of twoddle to adjust the speed. I guess I was thinking it might be for very short pieces and directed an audience who might not otherwise read.
Well, then it wouldn't need the words to move along, one at at time. Just a page on the screen, and the voice reads along. Basically, an audio-book, with a screen for anyone who wants to follow along. (Alternatively, buy the audio-book and the written version, and
boom, you've got it.)
I believe there are children's educational toys that work as you suggest, the child touches a word and the device says the word aloud, but I suspect they aren't used much once the child starts actually reading. The ones I've seen are sort of first-reader level. I would think that even for illiterate adults, it would be more annoying than helpful, and once you know how to read, the words would be a blur of screen changes.
Something that worked the other way: whole page on screen, touch a word and the device says it, would be helpful, for children and adults making forays into more advanced books, where they may run across the occasional new word. Can e-readers do this?