Because if I own your server, I can set it up so that it serves the good file to your pingdom-like service, and corrupted files to everyone else. Or more realis

Because if I own your server, I can set it up so that it serves the good file to your pingdom-like service, and corrupted files to everyone else. Or more realistically, I'd do something more targeted, like serving the bad files only to the ip block of my business competitors.