It's justified when a mage is part of some legitimate organization -- like, say, a country's military -- and using it to the ends that group is pursuing, while also under watch when doing so. Otherwise, I see it like civilians carrying military-grade weapons IRL: it should be banned, and offenders should get hit with serious penalties. I'm even okay with the Templars' kill-on-sight procedure.
That said, I was also willing to overlook Merrill's use of it, given Hawke assuming the responsibility that it doesn't get out-of-hand. Hawke's posse may not be a legitimate organization, but they get away with lots of things. Ya can't hate a clique of thug-killers.