I don't believe there are 'many' people who take MDMA daily for long periods of time, unless they only want negative side effects. They definitely aren't getting the same experience after the third day, regardless of how much they take. You only have so much serotonin. Perhaps these people are not actually taking what they think they are?
Here is an article about a decent study from the UK:
Anyways, I've seen many people claiming to have done this (or having friends who have done this) on Reddit:
https://www.reddit.com/r/Drugs/comments/45sgbe/daily_mdma_us... https://www.reddit.com/r/Drugs/comments/1ldmka/so_really_how... https://www.reddit.com/r/MDMA/comments/2765jb/i_used_100mg_o... https://www.reddit.com/r/MDMA/comments/211jkj/ive_been_rolli...
Lots of these cases may be false, but there still seems to be at least a few real cases of this. Of course, that's no reason to make the substance illegal (and especially not a reason to stop studies attempting to use it). Better regulation as a controlled substance is probably the best solution (i.e. enforcing the 2-3 month rule by only allowing MDMA to be purchased at those intervals).
There are also cases of individuals who believe they need to eat glass on a daily basis.
Anyone can convince themselves of anything if they want to believe it badly enough. I would be willing to bet that most of these chronic users would not exist in the first place had they sufficient knowledge about the drug.
I've heard of many people abusing alcohol, with daily use for long periods of time (all of which have reported memory issues and other permanent problems).
Still legal though.
2. When speaking about potential for abuse, it's worth making a distinction between drugs which cause dependency and/or chemical craving, and drugs that don't. You can develop habits without dependence or craving, but that can apply to almost anything, and is much less of an indicator that the thing is harmful. MDMA certainly had Prentiss for habitual use, usually as part of a multiple-drug-abuse condition, but in and of itself does not have dependence or craving properties.