It's considered best practice to use CSPRNGs for any cryptography, including salts.  Predictable salts may offer protection against rainbow tables..  Maybe it protects against "theoretical attacks"?

Take a look at this page:
http://books.google.com/books?id=QJNoykS0Tv4C&lpg=PT199&ots=JN9mj5AsnT&dq=salt+csprng&pg=PT199&redir_esc=y#v=onepage&q&f=false

It turns out that urandom is also cryptographically secure. The php function, mt_rand(), for example, is not.