If you're using the same salt for everything, you've very nearly defeated the purpose of using a salt. No salt means that a password hashes to the same thing,

If you're using the same salt for everything, you've very nearly defeated the purpose of using a salt. No salt means that a password hashes to the same thing, everywhere. A site specific salt means that someone can generate rainbow table to efficiently attack all accounts. A user specific salt means that generating rainbow tables is effectively the same (complexity) as brute-forcing.