The question of how many CPU cores are useful is always relevant to selecting the ideal system for your own particular workflow needs. For MPG that meant the 8-core 3.3 GHz system (special upgrade) because that particular CPU offers the fastest clock speed coupled with 8 cores, useful for intensive photography work.
The question is never cut and dried unless specific workflow is considered, because more CPU cores are generally a tradeoff with reduced (slower) CPU clock speed, so only a few slower CPU cores are used, then the workflow is slower on a machine with more cores!
There are other considerations too, such as background jobs while continuing to work interactively without disruption. Still other programs chew up CPU cores (“busy waiting”), but don’t actually get work done any faster ( software that does not scale).
Bottom line: on machines with 6/8/10/12 cores, can the software being used “feed the beast” efficiently enough for those cores to matter? This actually applies even more to high performance GPUs, which is why “faster” GPU choices are often not any faster: empty work queues.