They might also be useful on their own merits, as in, if you have severe depression and eat a bunch of mushrooms maybe you won't be so depressed anymore. Given the current state of anti-depressants, it certainly seems worth a shot.
You are correct. The first usages of MDMA (the defining ingredient in ecstasy) were in therapy.
If you have severe depression (or severe anything really), I would not recommend experimenting without an experienced guide. Think of it like like scuba diving or sky diving: first you learn how with those who already know.
There is a video of her speaking about what happened many years later
I think a huge part of having a guide the first few times is just so you experience it all being OK, so that if on a solo jaunt it's starting to not be OK you've got some understanding that it'll pass as, physiologically, so does the substance.