With the memory controller and graphics accelerator on the chip, it puts the bandwidth to and from CPU, graphics and memory at the performance limit. It can be found in various workstations and mainframes today. It will allow very easy hot swap, better streaming performance, and easy expansion. PCI will not be abandoned since it can be placed on one of the switched fabric links. Intel is slated to use a version of it for servers and high end workstations in a year or so.
Engineering is always a balancing act.
So this likely means a marginal increase in speed, and a dramatic increase in costs. That isn't bad on the high end, but would kill volume and the low end. I just don't think it is wise for Apple to try to customize in this way. Memory is becoming a limiting factor to processor performance. But that is only one way.