I got a short quote from this website, which basically sums up what I think:

http://cowichanvalley.cmha.bc.ca/get...self-harm#what

"It isn’t a mental illness—and in many cases, it isn’t a sign that someone has a mental illness. Instead, self-harm is usually a way to deal with difficult feelings or show distress."

To me, self-injury is a coping mechanism (a negative one, obviously). It can be a sign of a mental illness, but it doesn't have to be. I believe anorexia, bulimia, and other eating disorders are quite different than self-injury. I'm not trying to belittle self-injury by any means (I struggled with it in the past, and still have urges sometimes), but I don't believe it should be classified as a mental disorder on its own.