The weakness of this approach is that one never seems to be able to determine whether a particular question is non-threatening to a particular woo. I've occasionally asked a question which I thought was completely non-threatening, and they've bridled and gone all passive-aggressive on me. At which point one can only fall back on "smile and nod" and hope that any observers get the point.
I agree! Some people don't like questions, period! They regard their beliefs as opinions that they are entitled to. Why should they have to defend it, or be asked questions about it? The only questions they want to answer are from people who are potential believers, and who are genuinely interested and who they can
inform about their woo. It's hard for a skeptic to pretend interest of that kind, and I don't think you should either, to me that's close enough to lying to make me feel uncomfortable with it. So, if the woo isn't very daft, they will know rather quickly that you ask not to know more about the actual woo, but to make them question themselves and look critically at their own beliefs. Some welcome that, some (most?) do not.
Some know the technique very well, instinctively if nothing else, and wants to ask YOU these types of questions that will make YOU think, and will make YOU see what they see.
And some still are truly stu... not that very bright, and/or ignorant of very basic things, and you would have to explain so much background before starting to ask even these sort of questions, that it really isn't going to work.
Also, some that I have asked similar questions thinks it's a good way to debate them by turning every question back at you, without ever really answering them. Somewhat like this:
Q: "Why do you believe this?",
A: "Well, why do you believe what
you believe in? I probably believe what I do for the same reasons!"
And you don't want to be the one to avoid questions like that, so you do answer as best you can, and then you end up having to defend why you believe in reality,

while you will never quite get to their UFO, or Guardian Angel-beliefs being questioned in the same way. If you insist on getting back to their beliefs, they claim you are the one who doesn't want to be questioned.
There are many reasons that the innocent, polite questioning doesn't seem to work, even if you are friendly to each other, and discussing things just between the two of you. As someone mentioned above, it
never works well in a group of people, especially if the woo brought his/her beliefs up first, because then they feel they must defend themselves in front of people, and it all becomes a "save-face-situation", where no good arguments will get through.
But even when I have discussed such things with only me and a good friend, who claims to be open for discussing it - some/or all of the above problems arises anyway.
I have noticed, for example, that many of my friends who has wooish beliefs, have not really come to their beliefs by checking out every possible side of a certain subject and then choosing what they think sounds most logic or credible. They have gotten the idea from one, or a few similar, sources, and formed the belief in their heads with only, or mostly, the sort of input that spoke
for the idea. They thus think they have a good case for their idea, and then, of course, it's easy to claim you will be open to discussing it. When you then present a lot of opposing ideas that they now maybe hear for the first time, they are suddenly less open to listening. They feel you kind of snuck up upon them with it.
As friends it gets complicated. A friend's role is, foremost, to be supportive and acceptive, not to question, and I agree, but sometimes being supportive and acceptive of some things is actually doing a friend a disservice instead. The friends that I feel I should be able to be most open with about how I view the world, and that I should be able to be most honest with how I see their beliefs, are the ones who also will be the most hurt if I do.