For AES CBC encryption, whats the importance of the IV?

Question

What is the security threat of always using all zeroes for the IV? If it allows the encrypted text to be deciphered, how could an attacker do that?

UPDATE: So then,

Answer 1

If the IV is always zero, all messages with common text at the start (for example, emails with the same first two or three cipher blocks for the 'From ' or 'From: ' information) will be encrypted the same. This gives the attacker leverage over your key. That is unconditionally bad. As long as you use a reasonably random IV, the attacker loses that leverage. Don't risk it.