My dream Mac Pro consists of dual quad-core i5 2500k cpu's , 16gb of RAM (expandable to 32, for the "future"), 3 PCI slots and some drive bays.
Of course there can be a build option for the Xeon models. The performance gap between the MacPro's and the iMac's is so large that there could be an additional MacPro model aimed at those of us who need expandibility, performance, serious graphics cards, serious monitors, and don't have $3500 for a desktop. I'm a graphic/web designer who needs some power, but not Xeon power.
You can't have dual Intel processors without Xeons, so if you need 8 cores then yeah you do need Xeon power. Closest to dual i5 2500s would be dual Xeon E5-2643s which are 3.3Ghz quad core's, but with hyper-threading and 4mb more cache. They are $900 each though.