For those who are trying to provide homeland security, the major positive about living in a society with freedom of speech is that it may be easier to identify people who could be a threat. In a free society, people who might commit terrorist acts are free to voice their opinions. When they do, the security people can hear about it (for example, by looking at internet chat rooms) and have suspects to investigate.
The bad side of freedom of speech is that the security people cannot simply arrest people for advocating violence or anything like that. This prevents them from jailing people who might some day commit terrorist acts. They have to wait and hope that they can catch such people after they have started planning their terrorist attacks but before they have actually attacked.