For example, for all but the most extreme examples, how would you even authenticate something like that? Also, as much as theists hate to admit it, theories that they are hallucinations or neural misfires are valid points.
However, in order to think that they are hallucinations, you must have examined the accounts and found some evidence that they are hallucinations, such as a pre-existent psychological condition, a doctrinal inconsistency, or some other reason to discount it.
To be fair, theists also need evidence to claim that they are legitimate, but the point is that these accounts must be evaluated before any conclusion can be drawn. As long as there remains accounts of religious experiences which have not been investigated, then we must remain agnostic regarding their truth or falsity.
For example, imagine I had a powerful telescope. Through this telescope, you can look out into space and see Russell's Teapot. I would now be a believer in Russell's Teapot. To me, a random china teapot orbiting somewhere in between the Earth and the Sun would be a pretty extraordinary thing. I would likely invite someone else to look through the telescope and see what I see.
What if that person were to say to me, "No. I will not look. It is your burden to prove to me that the teapot is there. Until you prove it, I will assume it is a hallucination."
I would urge him again, "Look! The evidence is there, but you must look. I can't prove this to you, but you can prove it for yourself if you look."
It's true. I might be hallucinating, but the reasonable thing to do is to simply look through the telescope, rather than refuse until I can prove the Teapot's existence with a logical proof.