They really don't tell us about what 'MDMA' can do to people. They tell us what 'street-regulated ecstasy' can do to people. Everybody knows that ecstasy is notoriously impure/ not even MDMA. This is why it is hard for me to extrapolate anecdotes to myself (although I do take alot more caution than I would have). When my usage has all (bar my first time) been with tested MDMA, and I experience no negative side-effects, it leads me to the conclusion (perhaps falsely) that it is the adulterants mixed with MDMA that are the main dangers. Further, positive anecdotal evidence from users taking untested 'Mandy' which is, usually, just MDMA (or at least every batch I have tested was just MDMA) still report nothing negative despite taking it far more than once a month and often in high doses. But, as you quite rightly point out social bias is real and it would be unlikely for them to say what is really happening, so I would not hold that as 'strong' evidence. Despite taking more precautions than the average user, I still wouldn't hold my experiences as 'strong' evidence either. Arguably more valid? Maybe. But certainly not the truth.
Either way, it appears pretty clear that MDMA abuse can cause issues for those prone to it. But, it appears highly unlikely that once a month usage is going to realistically cause anything detrimental. Which is what the critics here seem to suggest as the global 'truth'. It's all fine and dandy reporting your own experience, but to suggest that it is the truth is too far. I mean I even read one paper regarding its use in therapy and they consider administering 2 times a month for a study, so unless they have seriously missed something, it appears unlikely. In their eyes, clinical use of doses ranging between 80-150mg appear safe enough two times a month. Sure, you could argue that the study is inherently flawed and biased toward MDMA, but I don't think they would have passed the stringent ethics procedure otherwise.