It’s true that language is defined by its usage, and words will naturally change their meanings (and have done so in the past). But on the other hand, that should not be taken as a licence to use whatever words you want to mean anything you want. The only reason we are able to understand each other at all is because of the consistency of language. When words are used to mean something different to usual, the meaning isn’t so much “changed” as “smeared”. You can imagine an extreme example that has some particular word whose meaning is different every time it is used. If someone says such a word, no meaning will be understood at all. By ‘misusing’ words, one strips away some of the meaning of the words for all others who wish to use them. So instead of “great”, “fantastic”, “awesome”, “amazing”, “sensational”, etc. all having different meanings, we just end up with a bunch of synonyms for “very good”. If we are to be able to express our thoughts to one another with words, the meanings of those words need to be more or less constant.

If we want to communicate complex ideas or anything with subtlety, we need words with clear and precise meanings. Vague language with smeared out meanings will not do. The rules of grammar and the subtle meanings of words allow for more powerful communication – greater depth of meaning, less ambiguities, stuff like that.

I’ve got an anecdote for you. A few months ago I went to a “philosophy night” at a local café. At this event people were meant to be discussing philosophical ideas so that we could all emerge wiser and more knowledgeable about the way the world works. Unfortunately, there were some important words which people understood to mean slightly different things. “Truth”, “true”, “fact”, “know”, “believe”, “objective”, “subjective”, … From my point of view we spent the night going around in circles because people couldn’t make their thoughts be understood, and the problem was that the meanings of these words was not precise enough. The meanings of the words was not precise enough because of how frequently they are misused. The meanings of those words were not stable enough to communicate with the precision that the philosophy required.

Finally, there is “right” and “wrong” in the sense that well defined rules of grammar do exist. If don’t follow the rules of grammar, maybe you aren’t evil but you are doing it wrong – that is to say you aren’t doing it in the way it is meant to be done.