I teach Latin, and one of my "rules" is to privilege form over vocabulary, that is, knowing the grammar trumps knowing the vocabulary word. Of course, that's an oversimplification on my part, but it's basically true. If a student who is translating Latin writes the English meaning over the Latin word, she loses necessary information: the English word does not express the grammatical function in the form like Latin does, as an inflected language. I encourage my upper level students to try to understand the grammar of a sentence before they start to look up vocabulary, since the forms should express which words are the subject, direct object, object of preposition, etc.

That said, I agree with you. When learning modern languages I want to build vocabulary--knowing the grammar is just not enough!