Actually my impression is that most people around here DO have 10 sq ft, or almost, per chicken in their run. Haven't actually tabulated it or anything though
If you're taking a poll, my one pen's run is 8 sq ft per chicken (but they have 15 sq ft per chicken indoors). The sussexes are using a temporary run while I get around to building two permanent roofed ones; the temporary one is 8 sq ft per chicken but the permanent runs will be 15-20 sq ft per chicken PLUS a fenced nonroofed area that will be used sometimes in a sparing-the-grass kind of way. (the sussexes have about 15 sq ft per chicken indoors, as well).
Unless they're getting out to free-range every day, I am really kind of skeptical of this "but I can squish them into less space and it'll be ok, right?" type thing. First, because while there is a random-chance element to whether or not you have problems, and certainly some people with some chickens keep them in very small areas without major problems of picking etc the chances of having that occur DO go up substantially as you crowd them more.
And second, because I've had chickens in all various size pens and stuff, and honest to gosh they seem a LOT happier in coops and runs that are a good bit larger than the 'conventional wisdom' of 4 sq ft indoors and 8-10 outdoors. By happier I mean they move around more in a relaxed happy curious way, and don't react so desperately when you give them something new for food or entertainment, and just generally seem to have more personal hobbies and personal life, if you know what I mean.
I'd say in general the bigger your coop, the more you can probably get away with fudging run size; also, the milder and more chicken-friendly your climate/weather, ditto. Still, there really IS value to more space. (Plus it allows you to add more chickens later; rebuilding is much more work and expense than building larger in the first place
)
JMHO,
Pat