I see both sides to the ‘its only the internet’ argument.
Sure, its all casual conversation, often supplemented by beer. However, as it is a forum, the only way that you are conveying yourself to a bunch of people who dont know you is through whatever makes it through your inebriated fingers and onto the internet. Personally I make an effort to convey myself as clearly and correctly as possible. Its one thing to not use apostrophes or to not capitalize things, or even make quickly typed spelling mistakes. Its entirely another to ‘correctly’ type out bad english.