You have a point, just because the mini is marketed as a budget machine and the displays are really meant for the pro line of computers. However, since Apple has no mid-range headless Mac, people who want both an Apple computer and Apple display have little choice. They can either get a Mac mini or a PowerMac G5--and for most people, even if they have $1000-$1300 to spend, that choice will be the Mac mini. In theory, Apple wants these people to buy the iMac G5, but if you want your computer and display separate so you can upgrade them separately, you're likely to go the cheaper route.
However, for people who really are buying the mini just because that's all they can afford, the Apple displays make no sense.
Because mini + cinema display = possibly the most styling desktop.
I say possibly because I haven't seen one yet. I am in the market for a new desktop this year. I would like a large display, but I am no fan of the AIO concept, or the white plastic. The computer would be in the corner of the living room, and aluminium is much more classic looking than the white plastic. As a powermac is far too big, heavy and expensive, and a powerbook is a lot more for little performance gain, the mini is the best option to run OSX on a cinema display. I am a little concerned about the ability of the mini to run the bigger screen, and of the future upgrade path. Given the failure of the cube, there is no guarantee of upgraded models being available in the future.
I am tossing up between this and an iMac. If there was a small form factor G5, then that would be a no brainer. I guess I will wait until I can try one, and see what Tiger Brings.