I used to think like you.
I used to think, "My god, these people get on a plane just to get miles. That is absolutely insane!" But then, I was two MR's short of 1k. I did two MR's, and voila, I'm now a 1k! It was the BEST thing I ever did with regards to air travel, hands down. The priorities and the upgrades are simply unbelievable. The service is awesome. I just call the 1k line, and I speak to a live native-english speaking human being who has my entire list of itineraries already on their computer screen. No waiting on voicemail for 25 minutes. No having them look for your itineraries and mixing up your last name. Unless it is TED, I am on business or first on every single flight (about 50,000 miles worth) all the way through July of this year!!
So yes, MR's do seem insane, I agree, but there is a little pot of gold waiting at the end for those who are willing to do it! ^