I use Tempests in almost every mission. They should be treated like a short range primary weapon, comparable to a quad Kayser, and you can rack up kills very quickly with them in a large swarm of enemies.
One issue with the Tempest is that the missile hardpoint locations on a fighter can make a big difference on how effective they are, since many fighters have hardpoints that are somewhat far apart and will result in one of the two Tempests (from dual fire mode) frequently missing small targets at close range. On the Myrmidon for example, the third bank is better than the other two.