The current resident pain-in-the-arse is getting to level 50 in Crysis 2 multiplayer. The rest of the online achievements i earned legit. And you know what really annoys me? I want to play the single player, but i can't, because of this sort of OCD thing that compels me to have all the multiplayer achievements in one perfect pile and all the singleplayer achievements in another perfect pile. They can't be mixed. WORLD WILL END.
Anyway i'm level 25 out of 50 now and the reason i forced myself to do it first is because i'm terrified of it becoming like every other online game that's no longer flavour of the month and having a dead online community, thus making it impossible to get the 1000g's on it. I'm also working on the Mortal Kombat online achievements such as win 10 ranked matches in a row. Don't even get me started on the rage i experience with that.
I believe the real reason achievement hunters (ones who take it seriously) do it is because we recognise completion above numerical value. For example, i've had people say "wow man i can't believe you did the 7 day survivor in Dead Rising, that's insane" or "how on earth did you do that final challenge on Vanquish", but when you see someone with over 200k gamerscore with duplicate games and "hannah montana" on their gamercard, they don't gain words of respect, they gain the ire of their peers because it's a cheap way of showing your "skill". I suppose it's also to do with having an element of perfectionism. "I must always strive for 50 / 50 achievements even when the game is no longer fun because everything must be NEAT UND TIDY."