Finished The Wind Through The Keyhole yesterday. A good story, and one I wished I had read before Wolves of the Calla. It is another one of Roland's flashbacks, though not so extended as that in Wizard and Glass. Maybe it gives only one bit of new information, but if that is the only new thing - it is a good one that sheds some more light on the character of Roland.
So why was it necessary for King to write a The Dark Tower 4.5? Because it bridges the two halfs of the series thematically, and the last three books is now more connected with the rest. It is not King going George Lucas, say thankee sai.