How can a PRO machine NOT have internal user expansion?
Because it has external user expansion, which brings a lot of new possibilities.
Now, you don't have to open the Mac Pro, uninstall cards if you want to use them with another computer. You can simply unplug the external extension chassis, and even use it with your MBP while on the go. And the new cylinder Mac Pro is also so small than you can carry it on a simple backpack (try to do that with the old cheese grater Mac Pro.
Apple is very clear about that. They believe the new Mac Pro is the future of Desktop computer. And I bet they're right.
And for the very demanding users (like me), we still have to rely on external drive enclosure, external card chassis, etc... because we can't fit everything in a Mac Pro tower, regardless how tall it would be. (I currently have over 12 hard-drive (all 2, 3 and 4TB) and they won't all fit in my current 8 core Mac Pro from 2010). So, I will still use my Drobo and NAS anyway... The good point now is that I can just carry on my drive enclosure and use it with my MBP when I'm away; something I couldn't easily do before when the drives where mounted in the Mac Pro.
To me, the new Mac Pro is just great size and specs. I'm not a big fan of the cylinder shape and I could have work with a square shape (such the new Airport Extreme). It would have been easier to mount it on a rack or so.
If only they would have a 12-core version with 64 to 128 GB RAM and 10 to 16TB SSD, then I would be good and more than happy... But until then, we will still have to use some expansion chassis such the new Magma Roben-3:
http://www.magma.com/roben