It is free software, and plays a crucial role in the operation of the Internet. And therein lies the problem. When a security problem turns up in BIND, the entire net is immediately vulnerable. In this respect, the net is a monoculture. Imagine the damage that could be done by a malign individual who is able to find and exploit a new BIND bug.
BIND shares a lot of characteristics with sendmail.
It is a piece of near-universal infrastructure which performs a crucial function. It is also too large and complex to ever be audited thoroughly. These mailers handle a small piece of the Internet's mail traffic, but they add a great deal of security and robustness to the system. BIND is not entirely without competition either. This DNS server was written by D. J. Bernstein, who also wrote qmail.