GREAT content in this thread. There are some wonderfully bright and helpful people in this community, and this thread is evidence to the generosity of their wise and experience-enlightened advise. Bravo.
Let me add a couple of additional thoughts. Do weeds "rob" the soil? Well, they certainly "borrow" but then later play it back as they decompose. So, yes, perhaps they "take" nutrients for a short time, but most of that is given back because "weeds" (READ: plants that are growing in the wrong place at the wrong time, not evil biological enemies) are annuals. In the grand scheme of geologic time, they are a mere blip on the time-line. They quickly grow and die seasonally—they ain't no redwoods.
I would argue that ANYTHING that captures energy and channels that energy into the soil-building process is, in the long term, contributing to soil health. Nutrients (commonly positively charged) are captured by negatively charged clay molecules. However, clay particles form long, tightly-packed chains that do not allow for water or anything else to flow through. Picture a stack of paper plates, one nesting tightly inside the next. Clay forms such tight bonds that even if their were wonderful nutrients deposited on the surface of the soil (animal poop, volcanic dust, nitrogen rich rotting bio-mass), those nutrients will not pass through and be captured by the negatively charged clay molecules. Unless something breaks up the tight clay soil, nutrients sit on the surface and wash off with every heavy rain.
Considering that 60% of a plants energy is used below the soil surface, whatever the weed may be "robbing" and transferring to above ground biomass is less than what it is pushing down into the soil. Between the sugary exudates that the roots are pumping into the soil to feed the biological community, and the actual root itself (which remains and slowly decomposes after the weed dies), that horrible weed is doing a tremendous amount of good below the soil surface.
So not only are weeds an additional layer of solar panels throughout the bio-system, capturing energy (BECAUSE, as we all know, the entire system is solar powered, so the more solar panels you have, the more energy you are going to capture), but just as important, their roots are punching through the soil profile, pumping exudates down into the soil and opening channels for surface nutrients to infiltrate.
On a micro scale with a one-season time frame, yes, weeds are robbers. But on a macro-scale with a long-term perspective, no, weeds are a tremendous asset to soil health and nutrient accumulation.