Papa sure is a man principles. It seems like no matter what Rob is thinking about, Papa has some words of wisdom to lay down to help him go about doing it right. Since the Pecks are Shakers living in a community of non-Shakers, we know that they don't mind being different and hanging on to their principles in the face of opposition or indifference. Facing up to things, living with modesty and humility, and working hard are all character traits that Papa values and that Rob, in turn, learns to put stock in. You may not agree with all the principles in A Day No Pigs Would Die (especially if you're a fan of women's lib), but you can't deny that they're there.
The Peck family's principles can be seen as a response to a difficult situation, sort of like making a virtue out of necessity. If they didn't have to work so hard and live so plainly, their principles would go out the window.
Following the principles of hard work and plain living only serve to make the Peck's miserable lives that much more miserable. They should give themselves a break and try to enjoy life as much as they can.