Here's an instance of using weeds,
I just moved into a spot where they had been weedwacking all the weeds, so the soil was basically just hard packed dirt.
I told the guy not to cut the weeds, and then we had the rainy season, and it's now pretty dense with a variety of plants, I'd guess there's 30 species at least.
So what good is that? Here's some benefits. (keep in mind I fully plan on planting the whole garden later, by laying preplanted slabs over the weeds, which will prevent weed seeds from being a pest later on)
To start with, it's bringing the soil back to life, it's a living mulch trapping moisture so insects can flourish, like earthworms who naturally till the soil for you.
This assists with the natural compost process within the soil. I even have moss growing between the weeds, and I live in a dry part of california, so this has really made a believer out of me.
The birds are attracted by the seeds and insects for food and plant material for nests (thistles seem bad but their flowers provide downy fluff for nests), and they in turn deposit bird guano and more seeds in said guano.
Various plants contribute in their own ways; Grasses aid in the aggregation aka tilth of the soil, clover provides nitrogen, and the higher the variety, the higher the likelihood you'll get a guild going, aka group of symbiotic plants that exchange services with each other, thus synergistically boosting the overall plant life.
And with my approach of placing preplanting slabs over the weeds, I avoid the problem of having to dig through the weed roots. Eventually the weeds and their roots will break down and provide food for the plants on top, and then I'll be able to dig deep holes for big plants that I started in pots.
So there's a perfect use for weeds. I think that once you begin planting you need to keep every inch of soil from getting weeds in it by planting dense ground cover in layers, like a 2" cover between a 4"cover between 6-8" cover.
The idea being no place for weeds to get sun. Of course some "weeds" are useful, like plantain, clover, dandelion. But once you plant them, they're not weeds, right?
