The Natural Language Institute method for language-learning, in a word, is combining immersive practice of reading, writing, listening, and speaking, using authentic materials and native speakers as teachers or guides.
Training all four language skills together in an acquisition program not only leads to more balanced and useful fluency, it also produces a synergistic effect. In other words, the combined result is greater than the sum of the parts. For example, while listening is obviously necessary to learn to speak, conversely, practicing speaking—while getting corrections or some type of feedback from native speakers (did they understand what you said?)—is essential for fully grasping the correct pronunciation of words and thus is very helpful in learning to accurately pick them out while listening to conversation.
Just to provide a couple more examples, writing is very helpful in improving speaking, since it is slow-motion practice in constructing sentences and expressing ideas in a foreign language. Reading not only provides a necessary basis for writing, it is also a very useful check on pronunciation and provides the broad vocabulary and accurate grammatical structures necessary for all three other skills.
Thus, my Mandarin Experiment and my French Fluency Recovery Project (FFRP) suffer from a serious flaw: they are one-sided listening programs and fail to benefit from the abovementioned synergies. In particular, as I watch Chinese shows and try to learn new words and expressions and consolidate those I already know, I miss very much having the opportunity to actually pronounce them to native Chinese speakers and get their corrections. If I had the opportunity to do so, I believe my listening comprehension would progress much more rapidly.
So why am I proceeding in a way that I myself consider far from ideal?
With regards to the Mandarin Experiment, the main reason is precisely the experimental nature of the project and its attempt to isolate a variable (can I learn to understand oral Mandarin just by watching and listening to authentic video?). There is, however, a second reason, which was one of the many inspirations for the Mandarin Experiment design and also happens to be the sole reason for my almost exclusive listening approach in the FFRP: listening is by far the easiest and most convenient of the four skills to practice.
I spend no money in my Mandarin experiment and have no hassle in arranging classes. There is no transportation involved. I can do it regardless of my energy level or motivation. I can profitably put in just five minutes or two straight hours—whatever fits in my day. For someone like me, who has an intense schedule with multiple time-consuming professional, academic, and personal commitments, this convenience can be a decisive factor in whether to even undertake and sustain a long-term language-acquisition project.
Likewise, the eight odd minutes I put into the FFRP are almost exclusively listening to French radio when I am about to go to sleep. I enjoy it tremendously and it is so easy to do. It also serves multiple purposes: not only am I practicing my French, I am getting world news from a fine media source and it is very effective in getting me to unwind and relax mentally as I prepare to sleep. In fact, there have been times that listening to French radio was the only way I could get myself to fall asleep!
In sum—experimental purposes aside—my projects reveal a big tradeoff between the ease and convenience of a listening-based acquisition approach and the effectiveness and synergy of a balanced program that combines the four language skills.