As someone who works in an area where language needs to be precise and unambigous, I find it depressing that David Crystal so energetically promotes his "if it feels good, do it" approach. I am tired of having to query badly written emails on which spending decisions will be based. The Lynne Truss approach leads to people understanding each other rather than being at cross-purposes.
I concur. If you allow words to change their tuesday then it may be that you will have heard exactly what I said but not have the faintest idea what I actually meant. A bit like listenign to Gordon Brown all the time.
Crystal doesn't promote an 'anything goes' approach, and in his original area of clinical linguistics I think he would agree with you that language does need to be precise and unambiguous. That said, language is not and never will be static, so people giving themselves ulcers over the 'decline' of English are setting themselves up for continual disappointment.
There is a clear distinction between someone telling you how to write in your own language (which is incredibly patronising) and someone upholding guidelines (which is only fair for universal understanding). If someone flouts the conventions of letter- and email-writing, you are perfectly entitled to respond to them with a catalogue of their errors and insist that the proper conventions are to be observed...however in informal writing and informal speech among friends, it is the most tedious thing in the world to be corrected on pronunciation, syntax and other things as you are talking, it is also considered rude in most circles. Spontaneous speech occurs as naturally as thought and for most people for most things, speech can leave the mouth before the brain kicks in, it would be unnatural for someone to complete a 'paragraph' of spontaneous speech without a single mistake.