I used to visit a forum where it was forbidden to post simple "thank you" or "great news" etc. messages. Such "airy" posts were not regarded as a valuable contribution to the thread and the mods came down hard by making a public declaration about, or banning, such users. The idea behind this was to minimise the number of posts that the mods had to moderate and also made the various membership levels more valid. It would be easy for me to find a random 100 posts and add a comment such as "Wow, that's interesting" which would immediately take my membership level up a couple of notches. However, I would know that I didn't really deserve it and a quick search of my posts would show that I'm new to the scene and definitely ask more questions than I answer.
It isn't just to limit the amount of posts to moderate. It's also to keep most "worthless" content out.
I know it's harsh words, but there's a reason why many hacking communities are like that. They don't want a database of 1 million posts saying "thank you". They would rather want 50'000 threads with good content, or 1 million posts with constructive feedback and critique, not necessarily both always.
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