If we were to make a habit of this type of signing I would 100% be in agreement with you. However, I believe there are exceptional occasions when such signings are justified, and this is one of them.
We're crying out for a dominant defender, somebody who, for his height, is really good in the air and a fantastic organiser. We also need a natural leader on the pitch. We need these qualities right now. A quick fix if you like. We don't need a long term fix as there's plenty of potential already within the club.
Williams ticks all the boxes and some. Over the last 3 seasons I don't think I've seen a defender who has been more influential against us; at times he has seemed to keep us out singlehandedly. In fact, I find it hard to name a single opposition player who has stood out more against us. He replicated that in the euros this summer, one of the stand out players for me in what was, admittedly, a very low quality tournament.
His experience and influence on the younger defenders (whether that include Stones or his replacement) would be invaluable. His inclusion, together with a quality GK and DM, is needed to solve the significant defensive problems that have been all too evident I our pre-season so far. So yes, I believe the £10m price tag is not only justified, but actually represents good value for money in the current market given what he brings to the table.