Unfortunately we still need some kind of signal that you're a unique person at signup to avoid infinite fraud (as it is we still get waves of thousands of accounts at a time who have worked out a way to get their hands on a pile of numbers and are finessing an attack on someone else via us).

You can delete the number once you've signed up, though of course that means that if you forget your password is reduces the channels by which we can validate that you're the same person! (this is also quite common: many HN users would be surprised how often people forget passwords)