Covered runs tend to run smaller and be more permanent, at least from what I've seen. I run my birds on a fenced in section of my garden (electric fence, no cover), and a couple times a year I move them and the fence to another section. So they're always doing something useful: either prepping new ground for future garden space (trampling down grass and shallow tilling), or cleaning up the remnants of the garden and eating the pests that hide in the soil at the end of the season, or just laying down fertilizer in the winter.
Whenever I see a static/permanent run, I can't help but think that if you have any sized garden, you're missing out on an opportunity to fertilize it, and you're also increasing your food costs by not providing an area where the chickens can forage. By always having them on grass or garden space, they always have access to greens and bugs, so I don't have to feed them as much grain. I have a small covered run attached to the coop but I only use it if I'm on vacation or need to separate out birds from the flock for some reason. It fills up with weeds every summer, but as soon as I let the birds into it, they trample it down to a dusty wasteland in under a week.
