Stupidity: Why weren't any of the guards thinking of running to the base of the crane to stop it? Surely there was a more than fair chance to capture Frypan as he had to stay up there and swing the bus to outside of the wall and lower it. Not only does that take a long time, he had to climb back down again.
Stupidity: They could've used the flying chopper thingy to kill off the armed guys when rescuing the kids off the train carriages. That way they could have made sure they got every single kid rather than rushing to just take the one containing Minho (which they messed up).
Answer: It's not so much that the rebels gain anything by destroying the city, it's just that they are untrained and driven by rage. They do not act as a military unit, they act more like a mob. They lack the discipline to strike only enemy combatants and instead cause chaos in the entire city. The point is that the rebels are acting no better, and in fact quite a bit worse, than the infected do.
BaconIsMyBFF