I often argue with the experts, either because I want to know exactly why we disagree, or because they consider their expert status more important than providing correct information. So no, I don't like the idea for such badges. Besides, I'm too easy to socially manipulate – that kind of sparkly things easily lead me astray –, but cannot in good conscience really claim to be an expert on anything; definitely don't have any accredited paperwork saying so. I really dislike the Stack Exchange voting mechanism, because it pushes most popular answers at the top, with the assumption that they are the most correct or most useful. Yet, popularity is no indication of correctness or usefulness; only popularity. I don't like popularity games.
What I would love to see, instead, is an Agree button. While functionality would be the same as with the Thanks button (which I admit I often use to indicate the combination of agreement and gratitude), its use and usefulness really would be orthogonal.
In many ways, such an "X, Y, and Z agree" list at the end of the post might actually serve the need Dave is wondering about this. Having the list of users who do actually agree would be essential for example new users to evaluate the value of such agreement, by simply clicking on the names and looking at those members most recent posts. No better way to evaluate someones output, than actually looking at said output in its proper context (reading entire threads discussing problems).
It would be an easy, quick way for other members to show their agreement, without filling the thread with "Agreed." type of posts (which we seem to avoid).
I often find myself wanting to just show that I agree with a post, because I myself have an earlier post on some aspect of the topic at hand, and simply just agree with a later post. Right now, I'm limited to "Thanking", or posting a silly "I agree." post, so I usually don't, unless I have additional related examples/suggestions/etc.
I'm hoping it would not be seen as a "DO THIS!" endorsement, but more like nods and agreement when discussing a problem over a coffee or other beverage among colleagues, in a poster session in an academic environment, or when helping a coworker with an annoying or problematic detail.
Of course, this has its own risks, and might not work too well either; I'm no expert.