Fair enough, if I had an imaginary friend I wouldn't want to report it to a shrink. I got hung up on technicalities and the point I should have been focusing on is whether entertaining one specific delusion is likely to result in other symptoms of schizophrenia that are more directly harmful.
See my take on that here.
Many people suffering from hearing voices etc. do realize those "aren't real", which doesn't in itself enable them to turn them off. If I were confident that you can untrain hallucinations (and strictly speaking thus get rid of a psychotic disorder NOS just by choosing to do so), switch them off with little effort, I would find tulpas to be harmless.
Not knowing much of anything about the tulpa community, a priori I would expect that a significant fraction of "imaginary friends" are more of a vivid imagination ty...
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.