That presupposes that we need to keep them anywhere at all.
It’s not that we need them. It’s that I don’t think people would be too happy with certain topics being completely off limits. If people hated them, they wouldn’t get such participation. Hell, if it’s an atheist witnessing thread, there will be Dopers who join in.
Exactly my point.
A lot of the participation is snark, and all of it, snark or not, is essentially telling the poster that he’s an idiot, and the poster ignores it and carries on, sometimes with enlarged or colorful fonts, because obviously, the larger or more colorful the font, the greater the truth (or truthiness, as the case may be). 
I can’t answer for others but for myself, I tend to think of GD as a mental battleground for ideas where facts and logic and reasoning are the weapons (ok ok, stop laughing, I know that’s not always true). Because of that perception though, I tend to think witnessing get an undeserved boost in validity when you can “debate” it GD, even if the debates are rarely more than “my special book says its true”. Putting it in IMHO or somewhere else relegates it to the proper, illogical, trivial deity gossip that it deserves. Kind of like putting the bible in the fiction section of the bookstore
An equally valid question is, what is to be gained by not deleting them outright?
But as that’s not going to happen, the question has to be, what is the most appropriate forum for it? “Witnessing” is, in the most literal sense, “seeing” or “observing”. Personally, I don’t see these “visions” as topics of debates. I think they are more appropriate to IMHO, or MPSIMS, in the sense of “I saw this, and I think it means that.” Whether anyone else agrees doesn’t seem to be of great interest to the OP. It’s more like sharing a revelation.
And like putting the Bible in the fiction section (or deliberately not capitalizing the title of a book), it’d be a cheap shot.
IMHO is for “less-than-cosmic topics.” Whatever the value, correctness, or logical coherence of religious witnessing, the topic is sufficiently “cosmic.” Whereas GD is for “religious debates and (if you feel you must) witnessing.” I don’t think it’s an accident that those are grouped together. Some witnessing is specifically intended to provoke religious debate or discussion, and even if it’s not, in GD you’re allowed to exercise your facts and logic and reasoning on it if you want to, or just ignore it if you prefer.
Politics and religion are the greatest of debates.
That’ll be true when women finally learn to pull the toilet seat up where it belongs when they’re done using it.
Which is why Witnessing threads should be in the Pit. That way posters can call religions and the religious idiots without having to cover it up in flowery language and the mods don’t have to pretend it’s not happening.
Come to think of it, that’s why everything should be in the Pit.