In the end, though, the rules do matter - it’s just that obeying them doesn’t. They need to be there to create a tension between conservatism and innovation. If the innovation continued unchecked, unmonitored by Susie Dent, then the language would fragment into thousands of mutually incomprehensible dialects.

http://www.guardian.co.uk/commentisfree/2010/jan/03/david-mitchell-english-language-grammar/print