The possibility that this completely ignores is pasting the password twice. The best passwords are randomly generated, lots of what looks like line noise, and very long. Computers are very good at making these passwords, so much so that programs exist solely for the purpose. Some password managers even have them built in. So, you want to create a new account (or change a password), what do you do? Randomly generate, then cut and paste twice (three times, if your password manager is not the same as your password generator). Disabling cut and paste kills this process, leading to the possibility of typos or weak passwords, thereby decreasing security. And, NB, not all password managers can autofill despite the HTML, and quite frankly, they shouldn't have to work around this misfeature masquerading as security.