You could for example apply your argument to say "well, is the voice threatening to kill you only if you don't study for your test? If so, isn't the net effect beneficial, and as such it's not really a mental illness? If you like being motivated by your voices, you don't suffer from schizophrenia, that's only for people who dislike their voices."
If you're going to define schizophrenia as voices that are bad for the person, then that would mean that it's only for people who dislike their voices (and are not deluded about whether the voices are a net benefit).
Voices threatening to kill you if you don't achieve your goals also doesn't seem like a good example of a net benefit - that would cause a lot of stress, so it might not actually be beneficial. It's also not typical behavior for tulpas, based on the conversations in the tulpa subreddit. Voices that annoy you when you don't work or try to influence your behavior with (simulated?) social pressure would probably be more typical.
Anyway... I'm trying to figure out where exactly we disagree. After thinking about it, I think I "downvote" mental disorders for being in the "bad for you" category rather than the "abnormal mental things" category, and the "mental disorder" category is more like a big warning sign to check how bad it is for people. Tulpas look like something to be really, really careful about because they're in the "abnormal mental things" category (and also the "not well understood yet" category), but the people on the tulpa subreddit don't seem unhappy or frustrated, so I haven't added many "bad for you" downvotes.
I've also got some evidence indicating that they're at least not horrible:
This doesn't add up to enough to make me anywhere near certain - I'm still very suspicious about this being safe, and it seems like it would have to be taking up some of your cognitive resources. But it might be worth investigating (mainly the non-hallucination parts - being able to see the tulpa doesn't seem that useful), since human brains are better at thinking about people than most other things.
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.