I have never done a "pure" mileage run where the sole purpose was just to fly and not actually do anything at the destination.

I have taken several trips (usually in December or February) that were spurred on by the mileage programs. The goal was to qualify for a status, earn some big bonus, or even take advantage of one last trip at a status I was about to lose. I've done a couple of $350 R/T's to Europe and $150 R/T's to the West Coast like this - but I always spent whatever time I could at the destination (even if only 1 night in the case of the West Coast).