I think the difficulty is that an s:decorateAll has no way to know how you want things decorated. So many possibilities in complex pages that you probably need to provide some info on how to decorate. Where do you need to put that? Either in the s:decorateAll tag, which makes it rather difficult to maintain since it is info away from the nodes/inputelements/.. it is about. The second possibility is to put it near those elements. This is easier to maintain and is in fact what the s:decorate tag does.
Still this is just my wild guess