Partially it's a way to sell the quad cores that come off of assembly with only 3 cores working. Better than throwing them out I guess and they work perfectly, just only 3 cores. Intel and AMD both do similar things, that's why we have stuff like dual core semprons and celerons. They come off assembly and only half of the L2 cache is working. They just shut off the bad part and voila, sell a lower end cpu that works fine.
Haven't seen any benchmarks for the triple cores but it seems like a decent concept.