They're faster to type, and I suspect I find them more pleasant to read (this is hard to test). AIUI, IIRC and similar are good disclaimers to make, but they're basically boilerplate, and don't need to take up lots of space.

Similarly, compare "(c) 2015 philh" to "this post is copyrighted by philh, as of 2015".

That's because you find them obscure. If your community used them more frequently you would just read them in place, like people do with IANAL, LGTM, AFAIK, etc.

Incidentally, I didn't understand this one, but I don't blame OP (oops, I mean 'original poster') for misunderstanding the linguistic norms of HN (hacker news).

They're a significant indicator that you're reading a message from someone who doesn't mind inconveniencing an unknown number of people to gain a negligible benefit for themselves. :) Or, at least, this is what I remind myself when I catch myself using them. Typing is not a hardship. I spend time trying to come up with the absolute clearest way of writing what I'm trying to say, so why would I then take a step backwards (potentially) in clarity just to save myself three seconds of typing?

fgsfds!