I can use any sort of console in Firebug, Developer Tools, etc. to access any Javascript included in a page and make calls to Javascript functions without a problem.
If, on a SharePoint page, I use the Client Object Model, doesn't this pose a massive threat to the SharePoint site by exposing service calls that allow you to completely interrogate and manipulate SharePoint objects?
What's stopping a visitor from firing up a console in the browser and doing damage?