Simply put, I don't think precepts really have the intended effect.
In an institutionalized setting, you go through the motions of "receiving precepts" and this somehow makes you into a field of merit. The laity then feel happy to bow down and make offerings, maybe getting some kind of religious high from the act of piety. However, once you know how monasticism really works and what goes on when laypeople are not around, then you see how efficacious precepts really are.
I'm well aware people are emotionally invested in a system that doesn't work as well as they believe it does, so my views are fringe and ultimately won't matter so much. In any case, as Buddhism develops in the west, I sincerely hope people exercise critical thinking and don't try to recreate failing Asian paradigms in the new cultures.
Also, if you look at things from an objective perspective, there are often curious historical examples of deviation from prescription. While Theravada has presumably always had precepts, for the longest time China and Japan got by in their monasticism with minimal and/or absolutely no Indian Vinaya system. I wrote something about this:http://huayanzang.blogspot.com/2013/06/ ... inaya.html
That means you can have vibrant and sustained Buddhism without strict adherence to scriptural precepts.