Is there any benefit to, as far as is humanly possible, total dedication to the precepts, in zen?
So total celibacy, vegetarianism and non violence to any life form, complete abstinence form alcohol, etc..
For the laity without a zen master.
"Perfect" virtue (when based on right view, which could be problematic with certain "Zen-views") is the condition for no remorse, no remorse... happiness, concentration, ... insight... liberation.
When focusing on the foundations, one works smart and straight forward till perfection. One hearing then the good teachings it might plopp.
Good associations/relations so that it might work.
(Note: not given to keep or give to keep in the wheel of trade, exchange, stacks... binding to worlds but for using as escape)
Yes. There is definitely much benefit.
I studied Zen for many years. And yet my practice was indeed strengthened when I started reading the Pali EBTs and observing more precepts. For example, I gave up dinner, cook for my wife and have the leftovers for lunch. And I regularly study the suttas. The suttas actually bring the Zen canon to life in a completely novel way.
Yes, of course.
However, in Zen, as in Vajrayana - and Mahayana in general - there is a strong idea of priorities. It exists in the Pali Canon and Theravada too, but there it is not so strongly emphasized, although they recognize it, too, without a doubt.
In the simplest example, if lying is required to save a life, one or the other concern may have to take precedence, depending on a situation.
In more interesting "Zen" situations, if overcoming your phobia or attachment requires breaking a minor precept, guess what wins.
The way I was trained, whenever you feel too holy and saintly, you should go get drunk or yell at someone etc. Basically, break the pattern. It requires increasingly fine discernment to perfect this skill, with different counter-acts and counter-counter-acts applicable in different situations, but you get the basic idea.
(Reminds me of stories... was it Milarepa? ... who burned the Dharma texts his student was obsessing about. Or that story about Chogyam Trungpa Rinpoche forcing a student to watch him throwing rocks at a cat. This last thing of course is extremely cruel, and I'm sure Rinpoche felt the cat's fear and pain deep in his heart. And yet, sometimes these are the kinds of things you have to do in order to help your students overcome some of their stronger attachments, especially attachments to high ethics and spirituality.)
So the above considerations put some limit on how closely you can uphold the precepts, but within those limits, yes absolutely, you should definitely keep them as pure as humanly possible.