Yea, I don't know why they even do this since there are very few flights w/in the system that actually are "direct" flights where a change of aircraft is truly a rare and unexpected event.
I remember you used to be able to count on these--in the mid 90's we would frequently do RST-MSP-YXE at a certain time of the day because it was the same DC9-30 on both flights... I think you could even leave your stuff on the plane @ MSP if you wanted.