Why is faith important?

[Darkstar]

I fail to see how having a knowledge of science invalidates their testimony.

Because when they are asked to prove that they really can use psychihc powers/summon spirits, etc. they either refuse or fail to do so.

By that standard human lack of understanding of the processes that existed before the big bang would invalidate our entire worldview and everything that we know today.

Lack of understanding? No. The problem is not that we don't know how people consort with demons (or whatever you mean to say), but that we can't find any evidence that they do at all in the first place. We cannot establish how it is done if it cannot be established that it is even done (by the person claiming they can do it, no less).

What you dismiss as superstition with no evidence

If it has no evidence, it is superstition.
...okay, here's the rest of that sentence:

other people have held to with the much stronger form of evidence of their testimony.

Actually, personal trstimony is one of the weakest forms of evidence there is. I could testify to literally anything right now, but I cannot prove my testimony is possibly true with science if it is impossible.

One asks for proof, they refuse. Another asks, and they try, but fail to give it. What is someone supposed to make of this? The stars were out of alignment, that's why my dowsing rod didn't work, right...
(By the way, do you believe dowsing rods work, and why do you believe the way you do?)