One thing that stands out in D.T. Suzuki's works about Zen Buddhism is that he all but ignores the pivotal role sitting Zen played in the Chinese and Japanese Zen tradition which we recognize in the school of Soto Zen or the same, Ts'ao-tsung Ch'an. Shedding more light on this, D.T. Suzuki believed that Rinzai Zen was truer to the core of Zen than was Soto Zen. Suzuki had few kind words for Dogen's Soto style of Zen calling it "taza-ism" (i.e., correct sitting-ism) which he believed gravitated towards philosophy in the form of nonduality insofar as sitting Zen attempted to synthesize, as one, both practice and realization.
I agree with D.T. Suzuki's need to be critical of Soto when it just amounts to taza-ism. But I cannot agree with his reasoning with regard to nonduality which for me is tangential if not confusing. As I read Zen literature, Soto or Ts'ao-tsung does stress taza-ism, to use Suzuki's term, which has little to do with the Zen of Sixth Patriarch Hui-neng or the tradition the latter came through which goes back to Bodhidharma of the Lanka School. Taza-ism, to be more precise, has to do with Ts'ao-tsung under Zen master Hung-chih's 'Silent Illumination' understanding of Zen which was truly about sitting. Worth adding, when Dogen came to China and studied Zen it was Zen master Hung-chih's Zen that he essentially studied, or the same, Silent Illumination Zen.
Briefly summarized, Silent Illumination Zen places a great deal of emphasis on physical sitting, advocating hours of this practice. Next, there is nothing heuristic in this practice. Sitting Zen is not a means to an end in the example of koan study. It is the end—an end founded on the belief that sentient beings are already enlightened so that there is no need to seek it. Just sit, in other words.
Here, I have to emphasize, is to be found the main weakness of taza-ism. The idea that sentient beings are already enlightened, that is say, are already Buddhas, is not found in any Buddhist Sutta or Sutra. In the Mahayana, Parinirvana Sutra, where it is first asserted that sentient beings have the Buddha-nature (Buddhata), this particular Sutra only means that sentient beings have the Buddha-nature potentially—never actually. In order to see and actualize their Buddha-nature, sentient beings must, in addition, undergo a purificatory process as the Buddha did when he was a Bodhisattva otherwise they will never become Buddhas, fully actualizing their Buddha-nature.
We can conclude that no amount of sitting can cause one to fully actualize their Buddha-nature, going from potentiality to actuality. Taza-ism, unfortunately, sacralizes the physical body's sitting posture. As a result it rises to importance over the very spirit which animates it. But traditional Zen is all about spirit, that is, Mind (S., citta) and the realization that Mind is Buddha (mind in this case being pure Mind). This idea is on virtually every page of early Zen literature where seldom is mentioned sitting if at all.
Finally, there remains the question of why sitting Zen is the popular form of Zen Buddhism. The answer is probably not that difficult if we understand how fixated our culture is to the body's importance. As we might expect in this culture, it is believed that sitting, being aware of just sitting (J. shikantaza), one does become a Buddha—or close to it. But all that this accomplishes is to hide the spirit, which is what Zen is all about.