A memory controller onboard of the processor increases system performance because it cuts down the commute time for electrons.
Intel does not integrate the memory controller. One reason is that memory standards change. Current Athlon computers, for instance, don't come with DDR II memory because the integrated memory controller connects to DDR I. Intel once tried to come out with a chip, Timna, that had an integrated memory controller that hooked up to Rambus. The flop of Rambus in the market led to the untimely demise of the chip.More details over at
Another reason is that Intel puts a lot of cache on its chips, which cuts down the need for an integrated memory controller, he said in an interview at the Intel Developer Forum.
"The only thing better than having an integrated memory controller on having the memory on chip," he said.