Interesting link. I have been mulling this over. Although historical precedents must not be ignored if language is not to be degraded - it cannot ultimately define it.
Language is not principally defined by grammar books , dictionaries or websites - it is defined by common usage and principally, oral usage. If this were not true how do you explain the use of language before the first dictionaries and grammar books were written ? What about the many languages that still exist today without a written form ? Language is spoken and then grammar books and dictionaries etc are written afterwards to help explain how the language works - not the other way round. After all the word "language " itself comes from the Latin lingua meaning "tongue".
There have been many divides in class and geographical location over the years with a variety of different English dialects and accents springing up as a result. None of them can objectively be said to be "correct" or "incorrect" . Language changes over time and due to the "law of babel" changes according to geographical location. Received pronunciation itself, which many view as the purest form of English, is actually further away in terms of pronunciation from the original English spoken by the Angles, than the English spoken in the North of England. What is grammatically correct now was not grammatically correct 300 years ago - yet that doesn't mean we are speaking incorrectly - it means the language changed. I cannot say that Americans don't speak "true" English because they have a strange accent or say cookies instead of biscuits - because the language changed.
Now there are some basic grammatical structures (like past simple etc) that do not change much and can be taught with definite meanings and rules, but when we get to the finer nuances of advanced English things are much more in a state of flux. Certainly it is too complex to be defined by written language which would essentially kill the language by trying to maintain it in a static condition.
So that leaves us with a slight quandary - how do we decide what is the correct use of an expression or word ? People often say that an expression is used wrongly - but this may mean that the use has changed slightly or that the person is using an antiquated expression who's original meaning or expression is no longer like the original, or that they simply use the expression differently in their part of the world . Dost thou doubt me ? for example would sound ridiculous in todays English but "do you doubt me" would have seemed most vulgar or even unintelligible a few centuries ago.
"I'm just after seeing Bill" would cause horror on this forum yet its common usage in Ireland.
This is why I think we cannot afford to define our oral language with written language because oral language changes much faster than written language (this may be changing as a result of the internet) - and as the wider world community seeks to learn English we have a responsibility to explain to our students (I am talking about more advanced students not beginners, intermediate etc) that the rules are sometimes just guidelines and that the rules of English are not set in stone.
Lets take the example of "beggars the question" - how do we decide what the correct use of this expression is. According to that article historically it was used to translate the concept of "petitio principii" but at some point somebody decided to use it to mean "raise the question" and now "begs the question" is in common use - and I can see why such a transition might have come about as a antidote to commonly presented disingenuous statements. The I suppose the crux of matter is whether the original change is due to complete ignorance and misunderstanding causing a clear break with the original meaning (in this case possibly) or a transition of meaning which has its roots in the original meaning (in this case again possibly).
Once its in common use however its it adopts a new meaning regardless of historical precedents - but it will still maintain the original meaning to those who study it to a deeper level . There are probably many expressions used in English whose erroneous origins are lost in the mists of time and yet whose use now is totally accepted. For a while pedants will complain that the use is incorrect - and they may well be right at first - but then the phrase will become incorporated into common usage. I guess this might be viewed as degradation - but I would say this depends on whether the changes in the language reduce the amount of concepts the language is capable of expressing.
Previously class was used to define the purer forms of English in the UK - however this is no longer appropriate as English is now an international language and the class system and the snobbery it entailed no longer have the power to shape language. So as well as common usage amongst the general population, we could look to popular authors , historical precedents, journalists, poets, historical figures and actors to guide us . We might also want to take into account the level of education of the person talking.
Basically language is defined unconsciously by the native speaking population , and language, especially its finer nuances, is a constantly morphing, geographically diverse phenomena that cannot be pinned down by the written word alone. The popularity of English worldwide means that increasingly people are looking to learn it and they are looking for hard and fast rules to help them do so. By catering to this need and presenting the language as something that we can learn in such a way I believe we are doing something which is extremely misleading. Non natives are therefore constantly surprised by native speakers speaking grammatically incorrectly. However this is not the case - the truth in many cases is that the language is changing and that the grammar books are out of date or simply not sophisticated enough to encompass the full range of the English language.
Most linguists argue that all language can be explained by a set of hidden grammatical rules. I am not sure if this is true - in the case of the current thread I believe that if it is true then the rules are a lot more sophisticated than the grammar books would have us believe.
As children we learn language without the help of grammar books or dictionaries. As adults we decide that we should learn from books . Why ? Some psychologists say that there is a part of the brain related to language acquisition that becomes dysfunctional after the age of about 7 or 8 and therefore it is impossible to learn in the same way as a child after this age. I doubt this for several reasons. I am beginning to think that we are going about language teaching completely the wrong way - I think first we should teach people to speak first and read and write second. I realise this is simply not possible in many places but when you've got things like
Language Learning with Livemocha | Learn a Language Online - Free! coming on the scene it's becoming more and more feasable.