I remember seeing my cousins ITA school books when I was young and wondering how the hell could it help to make it easier to learn to read English when half of it's written in a foregin language. Still, it doesn't seem to have done them any harm in the long run.
I'm pleased Al highlighted the fact that you can't be "a bit dyslexic" and that like dyscalculia and other related issues, while you can find ways to cope, they can't actually be cured. One of the psychologist's I've heard talk about these various types of developmental disorders believes that although we classify them as disorders in our civilised societies, they're really all part of our diverse human makeup and in more primitive times might allow some people to thrive as warriors or leaders while others organised more practical domestic issues or rebuilt Gordons.
The ITA thing does not resolve dyslexia. Spelling is not the issue. Its not recognizing the need to subdivide a word pixel in the first place. Its a bit like French. It is actually pretty useless to learn the names of things in French without also learning the defining initial Le or La. They are key to placing the word into the French language system of male/female tenses and be understood. Teaching is about recognizing these gatekeeping facters and including them in the initial process of learning. Sadly we are not all the same and kids respond to differing systems so one does not fit all. Once we have all the kids understanding a common code, we call ours English, you can move onto facts and concepts.
For writing it is assumed we all will right from left to right across the page, go back and start again. Why would we? Try Rongo rongo, it is still writing but the bonces do not understand it. Try giving it to some educated unusual and see what they come up with. Will not happen as those tittled as dyspraxic, autistic, dyslexic etc are meant to be daft.
Taking up on the other interesting factor I think the Egyptians, and there like, looked for the autistic and other oddballs. They took them into the priesthood, trained them and these people become human computers, memory banks and problem solving tools for the society. Controlled by the priesthood leaders. They would be somewhat untouchable and hidden away as they represented a valuable resource. This would mean writing was not really needed and only a break in continuity of the system threatened the collective facts and systems. Is this not why some societies collapsed so fast as a revolution or invasion destroyed the 'mentats' as Frank Herbert christened them in Dune. Without access to them the whole system regressed to an earlier and more basic standard of living, even to reversion to the stone age. After all we are fast approaching this very situation with computers. Turn them off and I really wonder if the younger generation could survive. I am pretty sure those over 50 would, save age starts to count against them.