That's a really good idea when you want to start the Burnt Land tactic.
If you're noticing you can't defend one of your frontairs in conflict during much time, under the pression and imminent advance of a powerful and encouraged enemy, who is decided to expulse you from the region at any price,
or also if you're noticing you haven't resources enough to support a so big empire, and those cities are not interesting to you anymore,
you can start your settlers production in every city in the region (if necessary resisting in the frontair at enemy's advance during some turns) until every city in the zone will start disappearing, but not in vane, since those settlers will be able to found them back, or to develop some other more useful and necessary region (like your motherland).
You won't loose when enemy breaks your military wall and opens a bridge foot, since you won't be giving cities, resources, fix positions, gold and technologies to conquer, and he will need to advancing a long to find something, what will give you a precious time. That nobody's land will be much slower and expensive to recolonize (since he probably didn't prevent that situation) and you could move and save your military units in the last turn by making them escape and run away towards a safe position, make them rest, reorganize them, and so prepare a delicious counter attack.
It's also good possibility as a punishment operation.
When a minor power defied you, offended you or just supposed a threat to your plans, and you've conquered a lot of it, until making him too weak, instead of just destroying him, and specially if you don't need so much terrain and cities, instead of giving them back to him or any other for free, you clean the territory (not in vane, then again) and make him repeat all the proccess from start. You're gaining a lot of advantage over him and he could be useful in future, by obstaculizing some enemy power's plans or growing up. Besides, that could serve as lesson for next time.
Definitely, with this idea and chance you've revealed to me, it won't be necessary to crack the game anymore to add diplomatic options like "gift/sell a city",

, or I'll have to switch to Civilization III, to increase fun hours.