I think there is value in having a consistent platform through out the enterprise. Not everybody is going to accomplish this, or go for it... but I do think those that use very large servers, are more likely to purchase the same "platform" for smaller servers and workstations just to maintain compatability.
As an example, the company I used to work for had several RS6000 servers used for the heavy lifting computing... They then put an RS6K workstation on the desk so that small jobs could run local on the desktop and the large jobs sent back to the server room.
The market is large and varied, and I do think there is a place for all of these products. The only question that remains is what volume and share each gets. I do know Itanium would be a very good fit for my old job of electronic design, while even the best X86 is woefully underpowered for it:-)
--Alan