Another thing is that even if you set a PIN, you'd still have to log into your account relatively regularly so that if you lose access to your number, you wouldn't lose an account. It's logical, given that numbers are reused... But that means that if you want to register without effectively tying your account to your ID (KYC when buying numbers is mandatory in a lot of the world, remember!), you'd have to pay for another phone bill (expensive given that the number's practically doing nothing!) or use a one-time rental... Which guess what, puts your account at constant risk!