Languages have basic rules. You can't pick and choose which ones apply to you. Proper writing/speaking is the basis of all civilized discourse.
Yes, but what is allowed in communication in any language is almost always more extensive than what prescriptivists will say.
For example, imagine say a native English speaker says:
"Yinz done did what yinz been told?[1]"
Even if you weren't very familiar with the use of yinz (although it's easier to understand in speech, given that it'll sound a bit like you+ones), you'd be able to comprehend the general sentiment, especially if you were even a little bit familiar with Pittsburghese or other Appalachian dialects.
This is at the core of language comprehension and usage; we are capable of understanding enormous linguistic variation within our own language. This cognitive ability has been taken advantage of by speakers of all languages, since at least the dawn of recorded history[2].
But there is such a thing as a sentence which would be wrong in English. I hesitate to include an example here, mostly because I frankly don't know much about non-North American English dialects, but I doubt that a sentence like "We are bites pizza and we have swallows the it" or something of the sort would be grammatical in any English dialect spoken by a native speaker. So yes, language has rules, but those rules are a lot more inclusive then one would expect based on the pronouncements of grammarians, and that goes for any language.
And language is a fundamentally oral business; the written form is a mere substitute for those funny noises you make on a everyday basis. There is no such thing as a capital letter when you talk. That's not to say that Evergreen shouldn't use capital letters, rather it is to say that you could understand them quite fine, given that the gap in understanding non-capital written English is much less than between GA, and say, AAVE [3] or Estuary English. And do we really want to exclude speakers of other dialects from our civilized discourse?
[1]This is actually a sentence I have heard someone say, for reference. The use of yinz as a 2nd person plural is mostly a Pittsburgh thing, but it has some peripheral presence in the rural Appalachians, including Western North Carolina, although it is being replaced by y'all. I remember this particular sentence in retrospect because the kid who produced it regularly used yinz, but was teased mercilessly for it and stopped using it by the end of the school year.
[2] Look up the history of the English word "like" for some fun variation; in Shakespeare's day it meant both "to like" and "to be pleasing to."
[3] Sometimes it seems like 50% of complaints about incorrect usage is people being racist about AAVE.
