This is what is typically known as an ur-text, namely a rough draft that has been extensively rewritten to produce the final work (though when I think of an ur-text, I usually think of the ancient world). Needless to say it is nowhere near as good as the original text, though for some reason people wanted Lewis Carol (or whatever his name is, and I can't be bothered looking it up at the moment) to publish the original story that he told Alice Liddel that day they were rowing down that river in Oxford (I know the river because I have been to Oxford, but I cannot remember the name of it – I think it is the Thames, but I could be completely and utterly wrong – and probably am).
I am not a big fan of ur-texts, unless of course a friend hands me one to read because they want to publish it (and so far I have only one friend who has actually published something, though he got me to critique the first book, but not the rest). The only ur-texts that I am actually interested in are the ancient ones, such as the ones that you find in ancient Babylon. For instance there are lots of different versions of the Epic of Gilgamesh, though there is one that is generally accepted as the authentic version. Then there are also theories of the ur-text to the Bible, but the thing is that we don't actually have them so any possible texts that arose is mere speculation.
However, when it comes to books like the Bible, the ur-text might actually be substantially bigger than the final proof that we have, and the reason for that is that the editors (Moses when it comes to the first five books) has only brought out the important points that they want to get across. However, I am now thinking that the original texts are not necessarily ur-texts but rather source documents.
As for modern literature, ur-texts can be useful for those who are extensively studying the particular text because it helps us understand how the polished version has come together. However, it can also be useful to aspiring writers in that they can see the rough draft that came before the final product. However some writers don't actually use ur-texts, such as Isaac Asimov, who didn't actually like to plan or rewrite any of his stories. In fact, the one story that he said he put a lot of effort into developing turned out to be really bad. In the end, as I say, each to their own.