That being said, you are using an internet forum which means that we do not get to hear inflection in your voice or pick up visual cues from your body language. That means that the only data we can use to form an impression of you is what you typed and how you typed it.
When you use poor grammar, it gives us the impression that you are some combination of ignorant, uneducated, lazy and ambilvalent. Honestly now, would you want to deal with people who meet those definitions?
(Please note, I am not saying that everyone who uses poor grammar is one of those things, I am saying that, by using poor grammar, they give the first impression that they are one of those things and first impressions are very important in that most people tend to accept/reject you based on them. Since that is the case, it is always in your own best interest to use proper grammar).