Imagine this: some "unstable" person books in for a flight with checked in luggage and blatantly walks onto another aircraft. There's explosives (or even something as small as a box of "Lucky Luke" matches in this person's luggage. The "unstable" one travels to ABC safely, but the luggage travels half-way to DEF before bringing the aircraft down. Therefore, it's a security risk.
It is if
two crews make a pig's ear of their headcounts. You DO do headcounts, yes ?