Well, the myth stems from the fact that about every pill or capsule you find will have MDMA-Hydrochloride (MDMA HCL) in it.
MDMA HCL is the most stable form of MDMA, as the free base of the drug is actually an oily liquid that's no so suited for consumption. Adding a salt like Hydrochloride causes the MDMA to bond to the HCL, creating a new stable crystal. In a sample of MDMA HCL, about 15% of it is actually made up of the salt, and the rest is MDMA.
Some people think this means that MDMA HCL can only be ~84% pure (impurities make up the other percentage btw), but that's simply not true. It means that only 84% of the sample is MDMA, but it is still 100% pure MDMA HCL which is what you want. Hydrochloride is not a cut, so the sample is still considered pure even though it's not technically 100% MDMA.
It would only even matter if you somehow got the other salt forms of MDMA, like MDMA Tartrate or MDMA Citrate that have different ratios of salt to MDMA.. but you will probably never see those as they are relatively unstable and absorb moisture, which MDMA HCL does not. Hydrochloride is an easier find for chemists as well, so it's really the only thing you're ever going to find.
There's a great thread on it from a while back if you care to search