a small rant
Anyone who's ever heard anyone talk about self-injury in anything approaching a professional capacity has probably heard it called a 'maladaptive coping mechanism', or words to that effect. It's difficult for anyone with any familiarity with the subject to deny that self-injury can be effective, but apparently it's also difficult to admit that it can be positive in some cases, hence the qualifier.
'It works, but it's not ideal' (what is?)
The meaning of 'maladaptive' in this context, on the occasions where it's deemed necessary to expand on at all, generally seems to fall into two categories that are also probably familiar: it doesn't solve the underlying problem, and it causes harm to the body. Of course, if it were supposed to solve the underlying problem we wouldn't call it a coping mechanism in the first place. Certainly it's possible for people to apply a coping mechanism to a problem that would be better off just being solved, but the problem hardly belongs to the coping mechanism. Since it's much better to cope with something that could be solved than not have a coping mechanism for something that can't be solved (or can't be solved immediately), better err on the side of having those mechanisms available. It feels like there's a belief that people's lives are nice and trouble-free by default, and therefore if anyone is experiencing a problem and not working towards solving it then they have only themselves to blame. In reality most people's lives feature some unresolved problems all the way through, but anything that brings attention to that, like deciding that a mere coping mechanism is good enough to live with, is subject to moralisation on the basis that it makes people who want to think the default state of the universe is pure bliss uncomfortable.
With respect to the second reason, it is unfortunately quite easy for people to believe that the body is sacred and inviolate by default. It makes the world safe, and any inconvenient counterexamples can, again, usually be blamed for their own misfortune. There can be no coherent notion of bodily autonomy that does not permit people to risk and harm their own bodies entirely at their discretion, but the idea of 'harm' is not necessarily even applicable to this situation. Unless someone wants to claim that socially acceptable body modification also qualifies as 'harm', then presumably it has to do with what the person in question wants and so self-injury often won't qualify either. Conversely, if you do want to claim that socially acceptable body modification is harm, then clearly the fact that something is 'harmful' by that definition is not sufficient reason to claim that it is also a bad thing.
Sometimes, if you're lucky, the term 'maladaptive' is used in a more technical sense: that it actually makes the underlying problem worse. Since self-injury is such a broad field, it's likely that this applies to some instances, and certain that it doesn't apply to others. Frequently both will be the case at different times for the same person. In this sense there is no such thing as a maladaptive coping mechanism in the abstract, only particular uses can be maladaptive. Labelling self-injury as categorically maladaptive, as seems to be done almost reflexively to avoid accusations that being nice to people who self-injure might be 'encouraging' something 'harmful', is - just like most categorisations - an excuse to stop thinking.