This is quite interesting - the increasing misuse of 'what' instead of 'which' in English ranks v. high on my list of linguistic irritations. Where there is a choice from a finate set of items, 'which' is always correct and 'what' is always wrong; but you hear this mistake more and more often in lazy speech. 'What flavour do you want?', 'what book are you reading', 'what category does this go in?' Etc etc - these are all wrong and sound terrible. In the context of the question i wonder if the que should rightly be a cual for the same reason or whether there is more flexibility in Spanish. The fact that DL wants to say 'what book...' makes me very suspicious about this.
The explanation that seems to be most often correct according to DL is that Qué is used before a noun as in "¿Qué libro vas a leer?" and cuál is used before ser, perhaps as in "¿Cuál es tu libro?". In a sentence that defines a term and uses ser then "qué" is correct rather that "cuál."
Well it's certainly a can of worms and that's for sure. The fact that I say 'lazy' and you say 'optional' highlights the situation quite well I think. We presumably agree that there are, well documented, rules on the usage of 'which' and 'what' and indeed 'qué' and 'cual' (in fact, I have just read the section in my Spanish text and it was very interesting, even though I know I won't remember all of it). The fact that rules exist and are quite clearly explained does mean that I can't happily endorse your term 'optional', but I have no reason to say that my opinion is more valid that yours. This said, one can only wonder at what point a line gets drawn. A test I like to apply to the 'language is a living thing let it breath' supporter is this. Is it not a corollary of your stance that you think that the teaching of correct grammar and indeed spelling to children is a waste of time, or perhaps a restriction on their freedom and the freedom of the whole language. Perhaps you wouldn't go that far. Anyway, assuming that most people would fall short of insisting on undiluted linguistic anarchy, I think you are saying that there are rules, but we must accept that they are going to get a bit knocked about from time to time; particularly if they are tricky to learn. If that is what you mean, then I feel sure that you are right.
I would say that, as language change generally occurs on a generational basis, a teacher should teach the rules that the majority of his or her generation has chosen to preserve. The students, as native speakers, then have the ability to preserve, reject, or change those rules in their own speech. Then, when their generation has entered a teaching role, they teach the rules that they have chosen to preserve. This goes for creating rules as well. English spelling was not standardized until around the 19th century when it became incorrect to use non-standard spellings. Likewise, the rule "don't end a sentence with a preposition" was fabricated by those who wanted to make English more like Latin. I think it's not that difficult rules are "knocked about" but rather that language is subject to the whims and judgments of new generations. Rules can be created and eliminated, and it's not the end of the world when they are. I doubt anyone today would mourn the loss of case markings, which were standard in pre-Norman English. A century from now, the loss of the which/what distinction will probably be little noted by anyone but linguists.
Language is by its very nature rule driven. The native speaker internalizes those rules automatically which is why when small children make errors they are most often applying the rule to one of the exceptions even though they may have not even thought about the rules or known that rules existed. The standardization of language rules has many purposes but the ones that I think are most important are those that involve providing communication among all people who speak a language. It is only the construction of standards that allows people to see and understand the variations. It also hopefully slows change somewhat which is paricularly helpful because change in different regions and groups and cultures would be unique and therefore language would become more diverse. Look at the different Spanishes let alone the creation of language families. Language WILL evolve but dragging ones feet can be helpful. The problem is that another purpose of standardizing language is to reinforce a sort of class system. People who speak a non standard version do not speak a less rule driven language are not stupider or even necessarily less educated but that is the perception. While I tend to be a heel dragger and have certainly armed my children with the language of the "educated and intellegent" I am aware of the bias. There are organizations for most languages that seek to determine what the rules are at any one point and that's about as good as it is going to get to solve that issue.
Languages are living entities that change to reflect the needs of their speakers. Rules exist to slow the drive for constant change.
But in fact rules are made to be broken.
Name one language rule that is not broken by native speakers who know the rules but choose not to follow them.
Language is not scientific truth. It is communication through consensus.
I essentially agree with everything you said. But you do make it sound like language change is a intentional act. Language change can be somewhat predictable, but most of the time you can't distinguish between word fads and permanent change. Although most people learn some grammar in school, they aren't even conscious of the rules most of the time. But they have been at least partially internalized because there are many errors made by foreign speakers that would never come out of the mouths of a native speaker. There seems to be more of a personality that each language develops that seems to be more of a factor in the development of a language than any speakers.
I feel I always send you links instead of explaining things, but there is always people that explain things better than me, so I hope you do not mind XD