If you've used any of the scholarly guides you'll know that it isn't really etched in stone - Modern Language Association (MLA), American Psychiatric Association (APA), Chicago Manual of Style, Associated Press (AP), or Turabian. They may all agree about a period inside or outside quotes in the instance discussed above, but they parse the rest of this stuff to within an inch of its life. And if you hop across the pond and look at European style manuals, a lot of it changes.

As long as people can make themselves understood, that is communication. The "rules of grammar" are arbitrary and are essentially "manners" of how one does it to be correct. Akin to using the correct fork to eat your salad.