As others have pointed out, the purpose of writing is for effective communications. If there is a breakdown in this communication, then several possible faults exist. There is a problem with the language used in its ability to convey information. Or there is a problem with one or both speakers in his/her competence. Or all three. I have written information that I could not convey, the reader could not understand, and the medium was inadequate for the representation.
There are several jokes that I'd like to share.
1) Don't teach a pig to sing. It annoys the pig and wastes your time.
2) Don't waste your time arguing with an idiot. He will drag you down to his level and beat you with experience.
Over all, my time is important. If you cannot understand what I am saying, my option might be to not communicate with you. Its not that I am a snob, but I want something more out of a situation. If I feel that I am spending too much time getting you up to speed, then there is something wrong.
For instance, I really dislike discussing religion with people who want to discuss religion. Fundamentally, they don't want to discuss religion, but rather, just want to either convert you to their mode of thinking or find out if you are in their in circle. While as I don't give a shit about their circle, it might be that being outside means that I am missing out on a lot of opportunities. Unfortunately, I don't like playing politics, so I'd rather work somewhere where I am doing as little politics as possible.
The other issue is that if your grammar is bad, then maybe so is your logic and math. By dealing with you, I am not growing and learning. Or not growing and learning in the way that I want to. I have served my time tutoring others, so I am not getting enough of a return on my efforts.