I run mobil 1 in my vehicles with 5-7K changes, and keep it topped off. Thus far haven't had ANY problems...
The 1996 4 cyl Mitsubishi Mirage has about 120K of mostly highway miles (15+ commute on an interstate to work), and apart from the interior starting to die, is doing okay. I just had the timing belt and water pump done, and I'm hoping to get another 10 years and 120K out of it.
The 1995 3/4 ton Chevy van was bought used with 44K on the clock... It's up to about 95K, and running fine. I'm thinking the engine may outlive the body. It gets changed on about 3K intervals, since it's primarily a tow vehicle.
I guess part of it is peace of mind - I could run regular dino oil and change every 3K, but as long as I'm running the mobil 1, I don't really worry about stretching to 6ish... Cost/mile is about the same, with half the time invested. Works for me.
Now, what I'd LIKE to do is install another cooler and filter on the van... The needle starts to creep up a little on really hot days, or towing up long hills. Never has overheated, or really even gotten uncomfortably near the red, but hey...
And what I'd like to see manufacturers offer is an electronic pre-start oil pump. You get in, flip a switch, wait five seconds for pressure to come up, and crank away...