I do think we should make an effort to know what the rules of grammar and syntax are and spell things correctly, it greatly improves the chances you'll be understood correctly, but I also think the grammar and spelling Nazis should be trashed and scorned. Language evolves, it's a hopeless undertaking trying to freeze it at a particular point, which is what such people are trying to do. I think the rate of change of languages has probably slowed considerably since the advent of printing, it serves to stabilize certain aspects of language, but it certainly hasn't stopped it. Consider, for instance, the difference in English between current usage and the King James Version of the Bible. The KJV is written in the English of 400 years ago, and much of its syntax is a little odd to modern ears, but it's still comprehensible. Go back just another 200 years or so, to Chaucer's English, and it's just about incomprehensible. And it was just after Chaucer's time that Gutenberg developed the printing press.
Besides, English is a sl u t of a language, it takes words from anywhere if they're useful, and as somebody once said, it's been known to chase other languages down the alley and mug them for new vocabulary. English vocabulary is huge and powerfully expressive and rich beyond imagining because of that, and out of respect for and love of it I do the best I can to use it well.