There are some apple and orange comparisons going on.
Treating phobias through BDSM could very much work. That would be exposure therapy. Trusting someone enough to exposure you to a fear in a safe environment, often gradually. Things like restraint, cages, blood, being dominated etc
Says she who has issues with being restrained, and has worked up to being restrained with emergency release that I can't use and cuffs tight enough that I can't pop out of them. That has taken a while
Still won't let them do it at the obg/gyn office though! No straps on my ankles, thank-you.
Cutting and other forms of self-harm are more likely the result of a mood disorder, the endorphins released by cutting create an "artificial" chemical high. By artificial I mean deliberately created. Like the runners high.
The treatment for this would not be exposure therapy. but probably physciatric and would require blood tests too, to ascertain if there is a chemical imbalance.
My 2 cents...