i think grammar is linked with circumstance. we don't worry about formal grammar so much in fights, drinking down the boozer and so on, but we are more careful in business letters and speeches. in fact there's probably quite a separate grammar for each situation, and learning how to use multiple grammars is a social skill.
"casting pearls before swine" as in using "octopodes" to people who wouldn't know the word, is just as much a gaff as the opposite, say, swearing in a job interview.
recently, for the first time, i've been in a position where i have to use japanese. the levels of politeness are very difficult, but i realised we have the same thing in english, just in japanese it's all tabulated.
more important than grammar rules, surely, is the ability to connect with your audience, whoever they might be.
after all, no one dies if you split an infinitive, so if it sounds better to you, why avoid it because someone decided it was a rule?
