Personally I cannot answer as to why the benchrest guys are using the WSM but I would say with a long action and a custom chamber it would offer the ability to seat the bullet out further. Also I believe that the short fat case makes more efficient use of the powder because with a shorter case you should be able to achieve a more uniform burn. It could be something different but this is what I understand to be the advantage of the "short and fat is where it's at" crowd. in a single shot setup fired from a short action they could also be a little more rigid.