Slang is a viable form of communication. Otherwise known as vernacular or colloquial speech, slang is a common form of everyday communication. Simple terms such as "sup" for "what's up" or "how are you" should be incorporated into everyday dictionary terms because of such wide usage. New words are created every day in living languages, so slang should be incorporated on a regular basis.
I think English is one of the strangest languages in the world, and certainly one of the hardest. This is due to all of its little quirks and whatnot, which are basically rules rather than commonsense. I think that if slang makes it easier for people to communicate, or if it's easier to learn, I see this as a positive.
As for slang in other languages, I imagine the same principle should apply.
accepting slang spelling, to me, is the same as accepting Ebonics as a legitimate form of language. I am very strongly opposed to this because as an african american who grew up Southwest Philadelphia, I see the question of whether we should accept ebonics as clear laziness on the part of the parents and the school systems.