There is also the possibility that it does help, but not strictly with the grammar or the mechanics of the actual language. I can imagine that learning a foreign language with any proficiency is as much about learning how to teach your brain to break free of the constraints of knowing only your first language. For most people, thoughts are directly connected to language and sentence structure. Perhaps learning another language teaches you to think more in the concepts and be more flexible with your thought structure than thinking simply within the natural constraints of your first language.

For instance, if you can't wrap your head around why a language may be missing 'basic' elements, you would never really master that language.