Back in August, Broadcast Engineering published an article written by Larry on this very same topic. In it, he talks about how the term license-free is often misperceived to mean that interference is impossible. The reality is that frequency coordination is needed to allow the maximum use of as many devices as possible without interfering with each other.
Human Element is Key:
Not only do you have production equipment on a single frequency, add in the tens of thousands of handheld devices (ie smartphones, etc.) of the spectators and you have no control over interference issues. The constant "chatter" these devices create raise the noise floor of the spectrum significantly. If you take these issues into consideration and communicate the potential issues and workarounds with all technicians, event vendors, participants, venue administration and IT departments ahead of time, you can assure that the proper equipment will be appropriately managed for optimum spectrum coordination.
Bottom line: Frequency coordination is the most effective way to protect yourself from interference.