You are not above everyone else just because you type out your sentences, first off. Stop acting like you are. It's annoying and I frown in your general direction.
Second, slang has a negative connotation, however that shouldn't influence your oppinion on it. Slang is informal language, and I doubt that ANY of you go around speaking formally all the time. It's not more mature to speak "formally," if anything it's more annoying. Internet slang is no different. XD is a form of informal language, because would you use a smily face on your college application? No, you would not.
By the way, nobody does the whole "Typ3 l13k d15" unless they're trying to be sarcastic or sound haXX0r, which they don't do normally, so it's not a very accurate example to be using.
Third, yes, I use slang, because I am a normal person, and I don't try to convince myself that I'm a step ahead of the game because I type properly. I use kk and brb on yahoo, because you know what? YOU CAN DO THAT. IT IS OKAY. Nobody doesn't know what those mean anymore.