If you have a deterministic screening procedure, then would-be terrorists can simply do a dry run to figure out whether they are going to be caught in the screening. As long as they can find enough white extremists (like the Boston Marathon bombers), they can guarantee getting through.
Random is trivially better than that. It has a chance of catching them. (If the plot involves many terrorists, the chance of catching someone becomes surprisingly good.)
If you have a deterministic screening procedure, then would-be terrorists can simply do a dry run to figure out whether they are going to be caught in the screening. As long as they can find enough white extremists (like the Boston Marathon bombers), they can guarantee getting through.
Random is trivially better than that. It has a chance of catching them. (If the plot involves many terrorists, the chance of catching someone becomes surprisingly good.)