Why obtain it on one airline then switch and not take advantage of it? The only value I could see is if your location/travel forces you to use multiple airlines. The reason they are called "loyalty" programs is the intention of rewarding loyalty.

If you can get the vast majority of travel done on one airline I would stay with it to maximize the benifit. However, you will find most of the responses you are about to recieve will have one thing in common "it depends on what you want/your situation"