Collisions between a directional link at short distance and other things using the spectrum are minisicule. It is like using megaphones to hold a conversation at a distance of say 10ft. It does not matter whether you are in a crowded area with everyone talking. The megaphones will drown out everything else. Using omnidirectional antennas is trying to talk without with out the megaphones. The difference in signal strength is what makes the wireless link with directional antennas more reliable than the one with omnidirectional antennas.