My point is there isn't enough backyard space to do this...and ignoring that, there are a lot of issues that come up from raising your own chickens. First, you have to feed them, which takes more time and money (and since they're fairly inefficient at converting protein, and will need supplemental feed, your total cost-per-egg will go up). You also need to clean up after them (hopefully at least a few times a week.)
You will now be attracting more predators into the city. Here in Greater Vancouver, there are an estimated 2-3,000 coyotes living in the area, and fortunately co-existence has been pretty good, but I could see other communities who would not be impressed with more species moving into the city. Racoons are pretty crafty, and are quite capable of getting into many enclosures, or finding when someone's been sloppy or careless in closing the enclosure (as happened to an acquaintance). This all breeds stupidity, an people end up setting traps in their backyards in an effort to 'control' the predators, and then instead of trapping predators, pets and children get caught in snares and leghold traps instead. (This also happens with regularity here in Vancouver.)
After a year, 'productivity' of chickens drops, and within 2 years you're getting an egg or two per chicken per week. So now you have to deal with this - either keeping them (and they live upwards of 10yrs), or disposing of them (I hate to think of what that means...) And then getting MORE chickens to replace them.
Chickens also get sick, get bugs, and need more care than just 'giving them food'. A few chickens in a block are pretty quiet, but if a large number of backyards in an area had chickens, the noise would become quite significant. Oh, and the stink. Doubtless some neighbours would spend as much time cleaning up, and so the stink from their manure would spread around the block as well..
I think there's a lot more to consider here.. I imagine there's a good reason people were eager to get hens out of their backyards in the first place, and find their eggs elsewhere. (Which brings us back full circle to how they're actually unnecessary, and companies like Hampton Creek are finding innovative ways to replace these things. ;)