In the Mustang, there is a noticeable performance difference when I fill with 91+ over 87, not necessarily in HP or MPG (I've noted maybe a 1 MPG gain), but more in throttle response and such. That alone is worth it to me. The car is much easier to drive around town as well.
I also don't drive 31k miles a year, so I can't really talk in terms of whether I'm burning money in that regard.
The other aspect I think has been overlooked here are the cleaning agents some companies add to premium. I am no liquid engineer, but if I am keeping a car for a long long time, I want the engine to run as efficiently for as long as possible.