I tend to use correct grammar and spelling when answering and asking questions. That could be a factor of me learning English as a second language, and being taught to speak it formally. Many people do use correct grammar and spelling though.
Actually, it's not fair to say no one uses correct spelling and grammar, because many people do. I think what you're referring to is really the state of education in these here United States, and quite frankly, it seems kind of dismal sometimes.