I pretty well agree with Tson. I tried mixing using these missiles in a few AI/me vs AI/me matches. (Sometimes the AI can be a bit stubborn about foolish choices.)
Sometimes the missiles did worse, sometimes better. Mostly better as support. I only found interceptors
under-powered.
Incendiary : With flanking, these have the possibility of being useful. Drilling was a good choice.
E.M.P. : Useful as supporting fire. Sometimes they're the right choice, other times not.
Dumbfire : Powerful, yes, but their slowness kept them from hitting smaller targets, and doing concentrated damage. Again, their usefulness variable depending on the situation.
Interceptor : Not so useful. They tend to steer around their target just as often as they hit it.
Clusters : Can't be sure as they currently directly damage shields. Effective against all but the fastest high firepower shieldless ships.
Most of my ships have pretty good PD. I found that the effectiveness of the missiles was fitting to the defenses placed.
Conclusion: Against fast, well thought out ships, purely missile ships will still lose. Missiles can, importantly, find their place as supporting fire; from a single ship or from a brawler. Missile ships will also be effective against turtle ships, railgun or no.