Grammar should be thought of as a series of rules that enable ideas to exist with as minimal impediment as possible. Notice that I’m not asserting that grammar should insure proper or correct usage; that would be an entirely subjective assertion. Reading dialogue, slang, or other forms of vernacular require deep understanding of grammar so that the reader can keep track of what’s being said. Put another way, the most powerful way to break a rule is for the breaker to fully understand what is being broken in the first place. Without understanding how things work, excellence can only happen by accident.