Actually, cities are valuable things. People almost never actually want the "no more city" result. - so what they're *really* good for is dancing on that hill for two-and-a-half days, at which point the invading army makes a few pertinent comments, and the city gives up and lets the invading armies in, while a runner dashes up to the top of the hill to tell the mages to call it off... which means in turn that fortress walls are no longer particularly effective as defenses when your enemies have siege-capable wizards on tap, unless they can field some sort of highly effective anti-wizard defense... which means, again, that as a reasonably-sized empire, you have to be *able* to field siege mages, but you don't actually wind up doing it that often... which means there's a definite niche for brave young grifters who can *pretend* to be fully capable siege-magii, and doesn't *that* offer all sorts of entertaining story possibilities?
Edit: also, I gotta think that the really major cities are largely immune to this sort of stuff. After all, while it may be possible to make cities go away by dancing on nearby hills with what you can bring with you, it's got to be easier (and faster) to make said nearby hills go away by dancing in specially prepared and consecrated focus rooms inside the city (with the targets pre-dialed-in and everything), and it probably doesn't even cost as much (siegemagii strike me as the sorts that enjoy being able to go home to their own well-appointed beds at night, rather than living in whatever style the army affords). Indeed, it makes trying to field a besieging army against them downright perilous. That little game of do-they-or-don't-they on whether or not a given city has siegemagii on tap is *also* an interesting bit for stories.
Worth noting that things that can work on that sort of a scale might well also be useful (and used) in pubic works - though the details on how the magic worked and what it was capable of doing would certainly have an influence on exactly how. At the very least, being able to make really big booms is helpful for things like creating mountain passes and digging (somewhat oversized) canals.
Edit: also, I gotta think that the really major cities are largely immune to this sort of stuff. After all, while it may be possible to make cities go away by dancing on nearby hills with what you can bring with you, it's got to be easier (and faster) to make said nearby hills go away by dancing in specially prepared and consecrated focus rooms inside the city (with the targets pre-dialed-in and everything), and it probably doesn't even cost as much (siegemagii strike me as the sorts that enjoy being able to go home to their own well-appointed beds at night, rather than living in whatever style the army affords). Indeed, it makes trying to field a besieging army against them downright perilous. That little game of do-they-or-don't-they on whether or not a given city has siegemagii on tap is *also* an interesting bit for stories.
Worth noting that things that can work on that sort of a scale might well also be useful (and used) in pubic works - though the details on how the magic worked and what it was capable of doing would certainly have an influence on exactly how. At the very least, being able to make really big booms is helpful for things like creating mountain passes and digging (somewhat oversized) canals.