I agree up to a point, Suibhne. But, tho anyone can, and should, be free to speak as they will, and in the tradition or dialect in which they feel most comfortable, over-much relativism, in this as in other contexts, can become counter-productive. A very fine English teacher I had at school all those years ago used to say that every pupil was bilingual — Classroom & Playground. There are standards to maintain and correct usages to encourage, which need not, however, be inhibiting to healthy, creative and interesting variations. An intelligent & well-adjusted person surely learns when to use which.