@manored,
In my own case, I thank posts because I appreciate the conversation more than the person that posts it. In some cases I also thank individual posts because the comment is very well informed or generally very good. That being said, there are many posts that lack those qualities. But I think that's a notion that many people on the forum do not understand.
Do initial "thankings" induce a sort of sheep mentality that lends to a post being thanked multiple times? I do not think so. If I come across a post which has been thanked multiple times, chances are it is written well and coherently and might tempt me to examine the whole argument more closely. That being said, "thanking" seems to be just a sort of advertisement. And if people want that effect, just put a whole bunch of emoticons in your post and grab the person's attention.
But thanked posts in my mind signals that the post may contain some level information which is interesting to read. Thus it may be part of a good, interesting, and philosophically inclined conversation, which is why (at least I would hope) we are all here on this forum. And I don't think we can fault some post or the poster for having more than two thank-you's because it has substantial content compared to others.
I cannot deny that I have seen quite a few posts thanked because there was some form of friendly understanding between two people which results in frequent thank-you's even though they may not be part of the thread. But isn't this the nature of academic circles though?
Also, since I consider this an interesting conversation, I have thanked everyone who has participated so far. But check out what happens (also keep in mind this is a set up for an experiment).