I think I found a counterexample to the common wisdom that more walls always create a stronger part.
The pictured S shape is 1.5mm thick, so printing with 2 walls leaves no room for infill. My testing wasn’t very rigorous, but it seems that the hybrid structure of walls + rectilinear infill is 10-20% more rigid than walls alone. The infill adds strength by cris-crossing between adjacent layers.
I think it’s fine to include a concentric top/bottom layer, but multiple identical layers weaken the part. I also tried 0 walls (infill only) and that was garbage.
Shouldn’t that be obvious? The same way a bundle of hair is softer than a rod of the same thickness. The extreme of this are sandwiches like carbon - lightweight core - carbon. There, the parts that experience the biggest forces (the outermost layer in tension) is extremely stiff, while the core only needs to withstand compression to keep both apart.