(Hello fellow DF'ers! I'm coming back to DF after a long hiatus, and although a very few of you might remember me from other forums, I'm rather new to the AT world...)
So my 2c's, here it goes
No habla espanol is one of the two or three sentences I know (and understand) in Spanish. So I guess I don't know Spanish at all, but I can manage to use this sentence appropriately whenever needed :-D
This doesn't mean that teaching several common sentences to beginners of the Spanish language is a bad idea, or that such teaching detracts from true learning. Of course, students get to learn vocabulary, and an appropriate amount of grammar. All the sentences they are being presented with helps them see the words and grammar in action. Even more so, it presents them with two additional value-adds that vocabulary+grammar alone don't offer -
i. It gives them something concrete to play with, and to parrot to their fellow students in a childish attempt at speaking. It reinforces familiarity with the language, and makes the learning process less abstract, and more approachable.
ii. Instead of having to learn all grammar through theoretical discourse, it allows the student to intuit, discern, and organically learn syntax and semantics of the new language they are attempting to master. It makes room for some learning through reverse engineering, and osmosis.
Which is why good language instruction almost always seems to incorporate vocabulary, grammar and common sentences. Of course, this is all moot without practice.
And there is always a chance that a student can end up like me - knowing only a few sentences, sometimes passing for a fluent speaker, but mostly not.