The ability to communicate is critical in nearly every aspect of life. In MA arcs one of the main ways you communicate with the players is through your written word - so in my opinion, the ability to write is critical to the enjoyment of the story.
Not everyone focuses on the writing, of course, and the other main method of communication - through the action and mission structure - doesn't have much if any focus on writing. Still, badly written text is going to alienate a large part of the playing audience. If they really didn't care about the story at all, there wouldn't be much reason not to go back to the farm of the month.
And for a while things were cold,
They were scared down in their holes
The forest that once was green
Was colored black by those killing machines
QR
I would say perfect spelling is not a requirement. Spelling well enough that errors do not significantly detract from the reading, however, is.
Yes, that "requirement" is rather vague, and is probably different for each person, but it's there. If you make enough errors in your writing that it hurts my eyes to read, there's a problem. If you write in dialect, or have the occasional simple typo (dropping a letter, esp. a silent one, often goes overlooked while reading), most people won't notice, or won't care.
The one peeve I do have about errors in writing, though, is making different errors every time. If you use "enough" four times in a paragraph and spell it four different ways, there's something wrong. If you use "enuogh" four times in paragraph, the same way each time... you're still wrong, but the consistency makes the error less jarring, and therefore less of a problem to read.
http://www.fimfiction.net/story/36641/My-Little-Exalt
How important? Depends on the audience. Personally, I can't stand errors in game text (and am still reporting some of the same ones, time after time, after five years) because they yank me out of the story. There are other people who won't be reading "all that text" no matter how carefully edited it is.
In my opinion, if you think you have a story worth telling, take the extra time to tell it well. You might try typing the text in a word processor first, checking for spelling and grammar, and then cutting and pasting into the Mission Architect.
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well thank you all for your response, i apreciate it, and i should also add, that some comments, were about the actual punctuation, ie..(( a [.] instead of a [,] and capitolize at the begining of the sentence. were i tend to use a capitol letter alot in some cases))
But as i told the commentors, that i would work on that, and i apreciated their comments, and would work on it. So i decided to post here and see what the consensus was, from the forum, compared from the 2 comments...
So Thank ya'll for your response they are apreciated, well written responses as well 
.. 
GL and Safe Hunting...
How important is it.?
I have made an MA arc. the main 2 critiques i have gotten, is a disertation on my gramatical errors, on spellin ((as i talk is say spellin) i know a "g" goes at the end.)) as an example...
Now im no collage major, nor did i ace english while in HS Back in the day... i dont kow a ton about comp. or programs except what i have learned through trial and error..i know enough to not feel swamped in tech talk. But its like i dropped the Holy Grail, if my Marc, reads like i talk, and not letter perfect.
I do understand where ppl are comming from and will strive to change my ways "typeing" wise, but not everyone an english major. so i dont see why it is such a Big issue. simply put i think it adds flav. to the talk your given, and read the story as such. tho i understand some just dont read between the lines that well, and or choose not to. anyway a slight rant on my part, but i am currious as to if it is a major deal, or if its just a picky few. I do welcome all response, as im not a whiner and get along with everyone and am pretty layed back and understanding, after all i am a dad of 3, and we arent talkin todlers lol...
the 2nd critique i get is on my custom group. i made minons that had Dark armour, so mez resistant, that was an issue, i changed that, i had a way to easy boss, so i changed that...
now its minions that team wipe and a killer boss, as put by another..so i am retooling the Marc will work on the custom group to balance them and make them an irritant, and not OP, (over powered)... i made the Marc i wanted to do, i hope it is enjoyed, Just real currious if the gramatical errors, is a Huge deal..i do speak english, and i type it as well, im sure yall will find numerous errors with this post. as i havent changed the way i type for this post..so i welcome all comments/flames/and anything else that might come my way...
I do apreciate all the response, good and bad, naturally..and TY for taking the time to read this post..