For reasons that eugenebrooks has covered (post #120), (post #123), namely being able to identify each node and prevent spoofing, simply filtering MAC addresses may not be enough. Both nodes that are supposed to be connected and/or a rouge node (say a malicious fan), could spoof a MAC address.
I agree that MAC filtering would probably suffice for differentiating between each robots traffic. However, I feel that wireless security is an important aspect to consider since, in the eventuality that there was a cracking attempt on the network, nobody would want to have their team and robot suffer. Network security at a FIRST event is once instance where I feel the Regan saying of "Trust but verify" is quite pertinent.
Also, a second on Bongle's post as well; security through obscurity is one of those funny things that I've read about working out poorly more than a few times.