With BCC, the recipient doesn't know all who got the message.
As you no doubt know, BCC stands for "Blind Carbon Copy", providing a way to send e-mails to people without revealing their e-mail addresses. This can be an aid for mass mailings without exposure, but also used to be sure an individual gets a note without the direct recipient's knowledge.
For every blow-up around poorly-trained marketeers who copy their entire customer base, or PR flacks who display all the intended reporters by dumping their contacts database in the TO or CC field, you have quieter mistakes that happen when the intended recipient who thought they were having a private conversation sees a third party jump into the discussion without warning. As the intended public recipient sees the third party elbow their way into the conversation, they may lose trust in the original sender, and wonder what other messages were being shared.
Often, those BCC'd blow the cover of the messages' routing.
Some e-mail clients (such as a BlackBerry) will automatically make it clear to the recipient that they were BCC'd on a conversation, which hints they should not reply all. But not all people are as discerning and recognize their role in the world of BCC to be a silent observer. I know that when I get a BCC message that any reply I provide goes to the originator and not to the others in the thread.
Given my goal to be public about communication, my own use of BCC is extremely infrequent. Due to the potential issues that can arise as trust is perceived to be broken, I tend to follow up a BCC note to the individual saying "You were BCC'd. Make sure not to reply."
If you are someone who tends to BCC individuals as a normal course of business, I have no doubts that you can get caught by this process. Not all recipients watch the To and CC fields closely, and they may expose your willingness to overshare. With so many other ways to have private conversations 1-1, and the ease of which forwarding prior conversations via e-mail can occur, it makes sense to get above board, and preserve the BCC option for simply obscuring mass mailings.