The thing is, people need to be smart enough to know the difference between right and wrong and good and bad. Some people are followers and will do just about anything another person asks them to. Others will stand up for what they believe in. Having an open mind is good like SpiralDancer said but when thousands of people end up following one crazy person with seriously demented ideas, it usually ends up bad. These people are like Hitler, Cult leaders, etc. They spread their ideas and unfortunately, there were actually hundreds of thousands of people dumb enough to do what he wanted and believed in what he did. People can be open minded but smart and logical at the same time.
My friends and I are rather fond of saying the following - "open-mindedness is all fine and good, but never be so open-minded that your brains fall out".
Keeping an open mind, means being open to possibilities other than what you might normally think of, or belive in. For the most part this is a good thing, because we don't know everything, and we never know when we might encounter someone who has something new to teach us. However if we take everything that is ever said to us at face value, and don't question it, or attempt to accertain the truth/legitimacy of it, then it's almost just as bad as being closed-minded, and - as you mentioned can actually cause harm.
Open minded doesnt mean acceptance. Just because youre willing to listen and entertain other ideas, doesnt mean you have to agree. Open minded people are willing to change their opinion when it seems right to them, without prejudice. It doesnt mean you have to agree with everything. That is wishy washy
Being open minded means questioning everything including what you know or believe. It means being open to the possibility that you may be wrong, or may not be taking the best path or action, and that other points of view may be right, or be in some way better. It doesn't mean be sucked in by everything. Consider everything. Look for evidence. Judge things on their merits. But don't simply accept everything.