I dont think such a switch is as feasible as it used to be.
Since XenForo is object oriented, while vBulletin3 was procedural, its a bit harder to have a blanket "disable". With procedural programming, you can just put a simple escape at the top and call it a day. With XenForo, you would pretty much have to escape every single hook, extension and object.
Its a lot easier to just disable it via the add-on options; as XenForo takes care of that stuff for you.