I believe there is a "correct" usage and an incorrect one. A simple example: Writed mine these bad-like. If you don't understand it instantly, you'll easily work out that it means I wrote this badly. Who's going to tell me that, even though, every word has been used "un-grammatically" it is not wrong, because it is intelligible. Words have particular forms to show what role the thing they refer to plays in the expression they are used in: I, me, mine; write, wrote written; this, these, etc.

English is a SVO language (subject, verb, object - the word order that is used to ensure understanding), and, because it has lost most of its inflections, it is important to say Janet loves John, where it is the girl who has affection for the boy, just as it is necessary to say John hates Janet where he (John) has an intense dislike for her. To say Janet hates John in those circumstances would convey precisely the wrong meaning. To labour my point, joining the two phrases together without paying attention to word order could result in the following preposterous statement: Janet loves John but Janet hates John. If we substitute pronouns, the nonsense is explained: she loves him but her hates he.

The necessity for a correct usage is most important where precision of meaning is required: scientific papers, enactments of laws, interpretation of contracts and so on. If it were not possible to describe a scientific experiment in precise terms, then it would be difficult to repeat it; if a law were expressed in vague terms, it could not be enforced; if a contract were unclear about what had been agreed to, how would you know if it had been properly performed or not?

This is not to say language must be codified and set in stone. Only a dead language does not change - cf. Latin. In any living language, attempts to prevent change, or to prevent certain types of change, will prove futile and are doomed to fail, even with government support, as with L'Académie française, which is a self-aggrandising body, overly conservative in outlook, and ineffective in what it does.