We really can't. Militaries in particular aren't into bioweapons. Too hard to aim. They're pretty much strictly the domain of terrorists, and terrorists don't have the resources to execute attacks on the bleeding edge of biotech.
Remember, no one has built a regular cell from scratch, much less come close to building a mirror cell.
For defensive purposes, FWIW. I suppose they might create some mirror molecules to test defenses, but I believe gus_massa's point stands that you don't need to generate full, functioning cells for that purpose.
Remember, no one has built a regular cell from scratch, much less come close to building a mirror cell.