Of course, those frag missiles pale in comparison to what heavy bolters can do
It all depends on the situation. Yeah, if the target is really spread out a heavy bolter has a
chance of getting more hits. On the other hand, if the enemy is clustered up, three missile launchers are more than capable of racking up more hits than three heavy bolters. It all depends on the target, the situation, and the terrain. There is something to be said about forcing your enemy to disperse, possible out of cover, and missiles do that better than many other weapons, certainly better than heavy bolters will. Also unlike h.bolters if you run out of infantry targets, or you're only going to get one model per blast, you can always switch to krak and at least have a
chance of hurting light armor, or annoying something heavy.