I think parish clusterings can be of use in deciding which parish is to close. I was a part of such a situation just about 7 years ago.

Church A was a modern building with a 900 or so family population. It had two Masses (Saturday night and Sunday at 11), a young and vibrant music director who was trying his very best with the resources and restrictions he had placed upon him, a pipe organ (that was rarely used at the Pastor's request) and a good sized social hall and rectory. The office space was minimal.

Church B was a modern building with a more traditional feel but had far less families (estimate 300). It had one Sunday early Mass, older musicians who were good at what they did but had the same restrictions, an electric organ (that was used regularly), a less than stellar social hall, decent offices, and a less than stellar rectory.

Church A folks would rarely attend events at Church B. Church B folks regularly attended events at Church A. Church A is in a desireable location with lots of traffic passing by it, while Church B is in an area that is in decay.

At a parish-wide town-hall meeting, it was decided that Church B would be closed about 3-4 years after the clustering. Most people from Church B came over to Church A and joined, some went elsewhere. The musicians from Church B retired willingly. The music library of Church B was taken to Church B.

It was clear that Church A would benefit from the closing of Church B, but the closing of Church A would be a disaster. This might not have been so clear to those making decisions had this clustering never taken place.