I think you are taking my argument too far. Valid input doesn't mean it is safe to use in any context. It just means that anything which is invalid at the start

I think you are taking my argument too far. Valid input doesn't mean it is safe to use in any context. It just means that anything which is invalid at the start shouldn't even have the chance to get in the system. You should still properly escape data when passed to another context, no doubt about it. I am also not suggesting "dirty" practices like replacing double quotes (Wordpress) or magically escaping quotes on input (PHP prior to... 5?). But if you know the input should be a number, allow only digits and '.' and clean everything else. And validate the range too! If you need a database ID, validate form and existence. If you need an e-mail address, run it through filter_var. Always. There is no reason not to.