I’m playing through New Vegas right now. I have one of the back buttons assigned to quicksave. I also changed the default camera button to one of the back buttons, because I don’t use it often enough to warrant it being on the bumpers. And I have just assigned one to toggle collision, because sometimes you need that when playing something built in the Gamebryo engine.
Trebuchets aren’t really a tool for defence. They have tremendous range and aren’t exactly speedy to load, aim, and launch.
Unless you meant defence in the same way that a country’s military operations are known as “defence forces” regardless of intent, in which case carry on.