No Consequence is right, of course.
I think it bears mentioning that, while neurotoxicity hasn't been established in humans at recreational doses, there are very clear and proven problems that emerge from rolling frequently. As No Consequence mentioned, you end up depleting your reserves of neurotransmitters (most critically serotonin) which take time to recover. Rolling also exposes your neurotransmitter receptors (again, most critically serotonin receptors) to a level of stimulation far higher than the norm. As a result, they downregulate to "tune out" the stimulation, which can take a long time to return to normal (and is even more impacted by rolling over and over again in a short period). Both serotonin depletion and receptor downregulation can cause the problems that No Consequence mentioned, namely: "random depression and anxiety and...memory problems/thought processing, etc."
Assuming that neurotoxicity does occur, rolling more frequently will expose you to the obvious risk of accumulating brain damage. I shy away from saying that brain damage is the cause of the problems that people regularly experience with Ecstasy -- neurotransmitter depletion and downregulation seem like more fitting explanations, since the symptoms usually go away with time.
Of course, MDMA isn't exactly the most fairly studied chemical...there're probably a lot of risks that have either been inflated, downplayed, or simply undiscovered/ignored in the war between prohibitionists and enthusiasts.
