It seems that many people who I would call "intellectually honest" end up developing speech patterns to avoid (1) speaking in absolutes; and (2) making statements of opinion articulated as statements of fact. An example of (1) is to use the words "many" or "most" instead of "all". An example of (2) is to start opinions with "I think".
Those speech patterns, though, can lead to writing which is bland and less than forceful, full of justifications with no interesting assertions to be found. One huge plus of this belief metadata, as you note, is that it frees the author to make bold and daring statements (and hopefully back them up) without committing to the truth of those statements. I think it can make for great writing – the style of strong and passionate assertion mixed with (via the disclaimer) the sensibilities of a reasoned opinion.
It's Obama vs. Trump.
Write down the answers in a few words, then slap them in brackets at the beginning, and there, you've created your first belief tag :).
This sort of matches the way I personally thing about information and my own opinions. Back in my head I always have this feeling of "degree of confidence", of how sure I am about what I'm saying, and it's often hard to express it in the speech itself. It fits perfectly as meta-data / qualification. Saying once, "this is a thought I'm playing with, I'm not sure either way" beats having to qualify every sentence with "maybe".
Also, I have most fun reading arguments that sound totally convincing to me but are tagged by author as something he doesn't have high confidence in. I end up having to figure out how this smart guy just convinced me of something he himself isn't sure about.
[0] - http://webcitation.org/6DuYcqyQ3