Vegemite girl is right- that is how it is intended to be used.
FWIW, it always annoys me when people in threads are employing PA tactics to shut the thread down or stop a branch-off discussion.
The "I'm bowing out" posts bother me a bit- if you're so disgusted with the thread, stop reading or posting, but don't detail in your post why the rest of us are so rude/disgusting/unfit to breathe and then pronounce that you are leaving...it comes off kind of badly.
The "I wonder when the mods will finally close this" posts also bother me- if you think it should be shut down, why are you still posting in it? Report it to the mods if it is a problem, stay away if it isn't...
I think when people actually misuse the bean dip phrase it can come across as rude or snarky, but it can be done in a good way. I remember a thread last year in which we were discussing some unusual spelling choices for baby names. One poster mentioned that she had chosen a name for her child with an unusual spelling, and she was instantly inundated with posts berating her for her choice, etc. She responded completely graciously at first, but then it got to be too much. The "bean dip" phrase there would have been totally appropriate, because people repeating the same point again and again, without bothering to read the whole thread or look at other responses, becomes very tiresome and the bean dip phrase works well.
As in life, you have to judge what is going on. Personally, I would be very careful about using the bean dip line unless I was the one under attack, but if it was a discussion involving two or three different posters then I could see any one of them throwing it out to defuse the situation.
The "I'm out of this discussion" thing doesn't bother me too much, because I don't see it as a way of saying "Goodbye Cruel Forum", I see it along the lines of "Ok, we'll have to agree to disagree, and I don't see any point in continuing because we're going in circles." Sometimes a discussion gets too tense, or a little too personal, it can also be a way of saying "Ok, I'm not going to be able to handle this politely, so I'm stepping out."
I agree with the bean-dip thing potentially becoming snarky, but that it can also be quite useful. I also agree with the posters who have said maybe it's better to be more straightforward. I do remember the thread you mention, and it did bother me quite a bit. I think when a thread gets that long, posters don't read all the pages, and repeat themselves. In that situation though, I don't recall if a mod was notified, but I do remember some posters saying something along the lines of "Let's lay off guys."
IRL I think the bean-dip thing is a way of humorously chilling out a tense situation. Sometimes it may backfire, sometimes it's better to be straightforward, but sometimes I think it works. I remember using it once myself - some close friends and I were having a political discussion, and things got very heated up and tense between two of them. I remember saying something that had NOTHING to do with the subject at hand, like "Sooo guys, read any good books lately." It was an obvious way of implying "Ok, this is getting too tense, maybe we need to relax a little and possibly change the subject." As I remember, after a moment, saying that made them stop for a second, and laugh at themselves, and then try to continue the discussion, albeit in a less angry way.