If I had existing PCI-X cards I wanted to install in the system, then yes I'd agree, but if you are building a new system there is no reason I can think of not to go all PCI-Express.
Anyway I agree 100% with your comment that this is a platform meant for a gaming+work system. Again it is like a Aston Martin or something. There are plenty of things just as fast, just not as nice. Unlike the Aston Martin though, this is not only high end but specialized. Specialized for the gamer, and CAD/3D Studio Max/Maya crowd.
Here's the thing, as somebody who uses dual CPU boxes at work, and has had them at home, this is a product I'd be interested in. But as that type of person, I do have rather nice PCIX cards...
but if you are building a new system there is no reason I can think of not to go all PCI-Express.
If you're on a budget yes, but PCIX is still pretty standard.
This just seems half assed. Either you're the type that has FB and PCIX, or you are not. Pick one side and go with it, why half ass it?