It shows your level of education and maturity. It's not hard to type "you". Typing "u" shows that you're lazy, inconsiderate, and stupid. It really is not difficult to type with proper grammar, there's no reason not to. The fact that it's a computer game is not relevant.
You can type with poor grammar if you're really that lazy/idiotic, but people like me are inclined not to listen to people with extremely poor typographical skills because those types of people typically can't be trusted with any sort of power.