I'll bite as well.

In addition to the reasoning given above, one has to consider that some of us travel to many distinct areas and travel enough in a year to make top tier on as many airlines as we wish (within reason), therefore there is the idea "not to put all ones eggs in one basket", which in my personal case has been a very valuable tool. I like enjoying my status on practically all the airlines I regularly travel on and their alliance partners. In some ways it is a great leveller, I can effectively shop for the best deals AND get the best service, its a win win and ultimately the exact opposite of what the carriers want.

Regarding hotels, your geographic example was extremely limited. I have been Hhonors Diamond for years, but there are some countries where Hilton has terrible penetration (China for example) but Starwood (for example) has always had more and better properties. There are also examples of cities where one chains properties are much better than the others, hence why it is handy to be top in more than one programme.