Nah parachutes should be enabled.
ZE flying world might be hurt by parachutes because the zombies can just kill all the humans by boding their boat but the only thing that you can do on ZE sorrento with parachutes is get on the final buildings roof without catching the secret boat and that is all.
Zombies gain no advantage unless the CT's are stupid enough not to shoot the parachuting zombies that try to get on your ship at the last second.
I do not see how parachutes on sorrento are bad.