I remember receiving a leaflet a few years ago from an insurance company. The message was their insurance policies were now being written in 'plain English', as opposed to a language most people can't understand.
When I write training materials, the guiding principle is to write for your target audience. The purpose is not to show how many long words you know, but for people to understand what they are reading. Surely that is the point of grammar, to help people understand.
People talk (or even argue) over the rules of grammar, but our language is constantly evolving and changing, and so I prefer to think of them as guidelines. Though I do note the dictionary defines grammar as a set or body of rules.
Publishing Perspectives has these thoughts on grammar today, and Sarah Duncan recently posted this guide to punctuation.