Interesting. We just had
a similar discussion in the UA forum.
I am in the camp that firmly believes that passengers should get the actual number of miles and segments that they fly. Other people who posted in the thread thought that we should be happy with the imaginary number of miles that we would have flown had a direct flight existed. That is stretching things a little too far for my imagination so I called UA and asked for the real mileage and they were happy to accomodate.
The difference in mileage for a flight SEA/IAD/FRA (actual flight) and SEA/FRA (imaginary flight) is over 1,200 miles, so it was worth the effort.