Merlin--
I sure as hell hope that occasional MDMA use doesn't cause noticable neurotoxicity, but I don't believe the evidence is as certain as you think. First off, MDMA neurotoxicity (as discussed above) is not simply a result of depleted serotonin levels. Nobody's quite sure exactly what happens, but something gets metabolized to a nasty oxidizing radical which screws up your cells. This type of damage could easily be cumulative.
It's also true that most studies have only really shown damage in heavy users. But that's partially because until recently, no one knew what kind of damage to look for; and also because the authors were typically aiming to demonstrate damage, not to ascertain what level is required for damage. Hopefully, more studies will address casual use in the future. It also remains a *very real* worry that noticable damage from MDMA-induced neurotoxicity might only become apparent with increasing age and the natural decline of brain systems.
My main worries stem from the fact that a known single neurotoxic oral dose in a monkey is 5mg/kg. Using standard interspecies scaling (weight_ratio^0.7), this is 1.4mg/kg in a 70kg human. Using the rat dose, we get 1.7mg/kg. Eg, for a ~150 lb person, we would have a neurotoxic single dose of 98 to 119mg, pretty much what's usually in one pill of E.
So put simply, extrapolating from the animal data, doing one pill of E will damage your brain.
On the other hand, studies in humans seem to show a more or less linear relation between total MDMA and impairment; and extrapolating (roughly!), one pill would cause the loss of 0.1% of the relevant serotonin receptors -- hardly noticable.
Of course, there may be more subtle damage not shown by this measure (technically 5-HIAA binding site density). And remember that there's usually individual variance in such things; some people may be far more susceptible to damage than others.
Anyways, on the bright side, no one's been able to show any real-world, testable, mental impairment from doing one or even a number of pills of E, at least not immediately, so you're (in my non-expert opinion) not gonna notice a thing unless you seriously abuse it. Still, I wouldn't feel too safe about it. If you're going to do it, it's probably a good idea to eat right, stock up on some good ol' vitamin C, and skip the big line of tweak that night.