If someone owns the right to the story and wants allow someone else to finish it that is their call. As a fan of the story you can read it or not. I personally don't consider them much more than sanctioned fan fiction. There are special cases like with WOT where the original writer left generous notes on how the story should end but without that it would just be Sanderson's idea of what would have happened and really wouldn't tell me anything about how Jordan wanted to end the story. I also give them a little more weight if the person writing the story knew the creator really well, preferable immediate family. A story is the product of the mind of the storyteller so if it didn't come from that mind it isn't the same story regardless of how good or bad it is or even if the writer was able to mimic the creator’s style.
I feel the same way about prequels. When the new Star Trek movie came I remember a lot of people saying how cool it was that now they "knew" how Spock and Kirk met. All I could think was "no Gene Roddenberry is dead and they are his characters and he is the only one that can create beginning for them". I could write a write a prequel and the only differences would my fan fiction was not sanctioned by his estate.