We had chickens who constantly escaped from their run. Bear in mind that you have to get them back into the coop every night before sundown or every raccoon/coyote in the neighborhood will treat your yard like a buffet. We lost a couple chickens that way; another got injured, and then her coop mates pecked her half to death because what do you know, chickens are assholes. I had to kill that hen myself just to put her out of her misery, and it was a pretty messy experience.
Anyhow. Chickens don't always get that they have to retreat to the coop. Which means you have to chase them into it. This is easy with a run. It is not easy without. It'll end up dominating your early evening, depending on how many chickens you're getting.
To top it off, chickens don't fly far or well, but they can fly far enough to get over a fence, depending on how resourceful they are about finding the highest take-off point. Then you'll have to chase your chickens through the streets.
Also, chickens obviously shit everywhere. And chicken shit is great for yards, but more by way of introducing it in an orderly manner via compost. If you just have chicken feces willy-nilly throughout the yard, it severely limits your ability to walk around out there, and if you have any kids who play in your yard, you have to worry a great deal more about salmonella and all that jazz. I once read that chickens are more likely to have salmonella in their poop if they've been eating rat turds; thus, if you're letting your chickens run around wherever the hell throughout the day, especially in an urban/suburban setting, they're more likely to find and eat said turds and again disseminate them throughout your relaxin' space as they please.