Shakespeare's grammar differs from modern grammar because language changes over time. Many of the rules of English grammar, syntax, spelling and punctuation were only codified during the nineteenth century when more of the populace became literate.
It's difficult to expound ideas brilliantly if one doesn't have the language skills to do so clearly. As Ken says, good grammar helps us to be better understood.
I do feel sympathy for people who are learning English as a second language, though. It is full of contradictory rules. But as someone who is learning a second language myself, other languages also have grammatical rules that seem contradictory.