It doesn't worry so much when cities sprawl onto deserts (sorry coyotes & cactus), it worries me more for example around here, east of Toronto, where cities routinely gobble up farmland. There are so-called agricultural zones but nowhere near enough ag land, IMO, is being retained for food production.
Society already knows that chemical and genetic intensification of yields--a common stated remedy to having less farmland--is hard on the environment (poison run-off, unknown cross-breeding issues, decreased soil tilth and increased erosion), so what the heck are the planners thinking? Exactly what do they think their grandchildren are going to eat? Now you've got me started...