I think that you've hit the crux of the argument for the mini-tower. These aren't "pro machines" with "maximum power" for the "pro target market".
The mini-tower would be for people who could get by with an Imac, but don't want an all-in-one for any of the good reasons not to get an all-in-one. Or any of the good reasons not to get an Imac....
It would be for people who need something more than an Imac (second or third hard drive or optical, quad core, more RAM, expandable graphics, extra PCIe slots...), but who don't need the humonguous expensive Mac Pro maxi-tower.
It would be for people who don't live in McMansions, and need a more compact system for an office or dorm room or studio apartment.
It could even be for the many professionals who truly can't use more than a couple of cores today, or who don't need the expandability of the Mac Pro.
(MM - you have blinders on as far as the "pro apps" using lots of cores. The media apps that are important to you are nicely threaded, but many other pro apps are not and some of them may never be. Look at the set of Photoshop filters that are not MP-aware.
Many tasks and programs are sequential in nature, and are difficult or impossible to parallelize to a great extent. Look at
Amdahl's Law for more information on the limits of parallelization.)
HP can put a compact quad core, 3 GiB, Gigabit ethernet system on the shelves for $800. Why can't Apple give people a choice between the limitations of the Imac and the overkill of the Mac Pro. (Especially since so many people are saying "all 8-core" for the new Mac Pro.)
ps: Since "CPU" and "processor" are ambiguous, I use the unambiguous "single socket" to refer to a desktop Conroe/Kentsfield system.