Let's get this straight. An iMac differs from a Mac Pro on these areas:
Expandability in terms of PCI-E. (iMac doesn't have PCI-E, MBP's at least have one express card slot)
RAM amount (iMac maxed out at 16GB)
GPU speed and multiple displays (iMacs have slower GPU's if you are a gamer, but for most professionals, this is irrelevant. If your apps can make use of Open CL, then it'll matter but we haven't seen many examples of this yet. Multiple displays are an option for TB equipped iMacs, but then you can't use the TB port for other stuff)
Core amount (iMacs maxed out at 8 threads where Mac Pro's are at 24 threads)
And that's it.
It has more internal storage, but since TB and USB 3.0 have enough bandwidth, external storage runs as fast as internal storage now. Unless you are stripping 10 SSD's to get 3.5 GB/sec ofc, then you need this through PCI-E and TB won't be enough.
So unless you actually make use of one or the other, the internals of an iMac isn't any "slower" than the internals of a Mac Pro.
The main point you miss is a lot of us don't need the monitor. Or don't want a built in monitor. We don't want to pay the premium for it as we have invested heavily in Cinema Displays for other displays we love.
Replacing your machine and replacing the display with it seems so wasteful.
No matter how you slice it, there's a gap in there product line.