I wouldn't buy it, I'd get it from the library.
But seriously, why not, I don't have anything the imagination of others. It feels unfair if a best-selling book is based heavily on something written by somebody else, but aren't most books that anyway.
I would regard such books as "Tolkien" because they're not, but at the moment I can't find a reason why I wouldn't touch them. If they're written, they might be worth reading.
Somehow this reminds me of Gone With the Wind's sequel, though I have read neither the original nor the sequel.