Good topic, Vaft.
I disagree. Certainly grammar and language should be allowed to develop (and they will develop, whether they're allowed to or not). But only developments that preserve or promote clarity and precision in communication should be encouraged as correct. Not all grammar is equal, and some developments in language make it harder, rather than easier, to clearly and precisely communicate ideas.
A simple example is two words evolving to mean the same thing, when they did not before. I think this should be discouraged, by and large. (What I mean precisely is that, if the new usage of one is considered "incorrect," then that should continue). Why? Because once the evolution is complete, and there are two words that mean the same thing, the language will now be less well adapted for communicating the two distinct ideas, and thus less well adapted to easy and clear communication, at all. A classic example, I would say, is "comprise" and "constitute." Comprise is a very useful word that doesn't have any very useful synonyms, but its meaning has been evolving toward "constitute." This should be resisted, because we need a word like comprise.