Almost any construction used naturally and widely by native speakers is "grammatical."
So if two contradictory constructions are used "naturally and widely" then both are correct?
I would argue that "irregardless" is not used naturally at all, it is used uncomfortably by people who want to sound well-spoken but don't quite know how. Same with "whom" used incorrectly and "I" being substituted for "we." This may be widespread, but it is not natural. It is the result of people trying to use the language correctly, but not actually knowing what the correct way is. Grammar rules don't exist just to annoy people and make school boring, they give us a framework for using and expanding our language. I agree that the flexibility and adaptability of English is one of its best characteristics, but there is a difference between a language developing and progressing to meet the needs of its speakers, and speakers mutilating a language because they aren't taught how to speak it well.
I really object to people being called idiots for the way they speak just because one read it was wrong in a fifth-grade grammar book.
I teach English to non-native speakers, so actually my knowledge of grammar comes from my training and the textbooks I use to teach. The grammar I teach is necessary and relevant to my students, and if you tried to tell them that it wasn't important, you wouldn't get very far.
Aren't there enough problems in the world without native speakers telling other native speakers that they can't speak their own language?

Native speakers are capable of making mistakes in their own language. This is true not just of English but of every language. Just because you grow up speaking a language doesn't mean that you know everything automatically or that you don't need to take the time to study and learn the formal conventions. I don't think anyone on this thread said that native English speakers can't speak English, only that there are some pervasive errors that are irritating, illogical, and easily avoided.