The usage of abbreviations for phrases and acronyms by computer users probably developed from a number of valid sources. One is out of chat and talk programs. In former days, connections were really slow enough that typing out whole phrases actually was pretty tedious, as you could usually figure out what the other person was saying before they finished the sentence. Even those who use chat today still use these to communicate faster since most people type slower than they talk.
Another source would be from command interfaces that used abbreviations for words (id est Unix, DOS, et cetera); those familiar with it don’t have a problem seeing a statement like ‘rm -rf /bin/laden’ [sup]*[/sup] as starting with “rEmOVE” (even though they wouldn’t pronounce it that way).
Of course the main reason behind that, and the likely reason abbreviations persist on message boards is as you said the tedium of typing the same phrases over and over. These communications are somewhere between chat and formal letter-writing; while there is not much need to type fast, people often do not feel like spending a lot of time typing, so slightly hurried and un-proofred responses are the result. Brevity is also favored, and longer posts (exempli gratia this one) are often skimmed. This is not so bad, as sometimes a long post written quickly tends to ramble and throw in completely off-topic things like jokes about terrorists or ingratiating self-reference.
I don’t think most people who use them, especially those here (your experiences elsewhere may be different) do so to intimidate the uninitiated, they’re just doing it to avoid typing the same lines over and over. Most of the time people here are trying to educate, not exclude, so someone (if not the poster themselves) will explain commonly used abbreviations. Those who bristle at requests for explanations have usually had to write out the explanations several times themselves and are sick of doing that. On occasion, explanatory threads can be found (usually in About This Message Board), if the context does not become any clearer.
There are some who wish to use it as a form of talking in code either for privacy (POS in one context, or completely strange phrases like you’ve got) or as a way to avoid using certain specific words (RTFM, or POS in another context). If it’s just to be exclusive, I wouldn’t think that they would want a lot of people to start using it, since that destroys the privacy of it.
I’m not saying that there might not be people who do what you describe, I’m just saying that most of the time I don’t think that’s the reason.
Explanations of the abbreviations used in this post (and other older ones) can be found at : The Canonical Abbreviation/Acronym List
[sup]*[/sup] This is a joke, but it is not original with me.