DHE: Vivid Hallucinations a Gateway Psychosis?

I've started listening to a DHE course. One of the early exercises in the course is about vividly hallucinating crosshairs. Like, hallucinating that there's actually a pair of crosshairs on the wall in such a way that you can't tell that there's not really a pair of crosshairs on the wall.

Just considering doing this raises in me a massive ecological objection: if you start doing that, what is there left to differentiate between fantasy and reality? What is there to keep you from becoming a full-blown schizophrenic?

More to the point, it seems that the near-universal inability of humans to do that kind of thing by default is probably in place for that very reason. It's for a similar reason, I assume, that we tend to experience massive amnesia about our dreams: keeping that split between fantasy and reality is extremely important to one's sanity (to say nothing of things like driving safety).

I considered going half-way and partially hallucinating things--you know, that half-translucent things. That seems more useful, anyway. Why are you going to want to hallucinate things that occlude your vision?

So anyway, I'm curious: Has anyone here taken the plunge? Hell, has anyone here even managed to do it at all?