I think I’ve heard the saying that you should never mix turrets and launchers. Is it really that bad if your ship can fit both, to mix them? For example, a Rokh with both heavy railguns and heavy missiles?
First, you want to fight at the optimal range of your weapons, so if you’re using different weapons you will use one of them sub-optimally.
Next, you want to use modules and rigs to increase the effectiveness of your weapon system. Better application, more damage and such. If you’re using different weapon systems, your modules and rigs only work for some of your weapons instead of all of them.
These are just guidelines, so if you can find a good reason to equip different weapon systems in a way that works for you, go ahead.
as a rule @Gerard_Amatin is on the money. but sometimes you’re options are limited, and getting some dps from launchers when you have no more available turret slots is better than getting nothing. So long as you only use mods / rigs / implants for your primary weapons systems
There are a few Minmatar Faction ships (Scythe Fleet Issue and Typhoon Fleet Issue) which support mixing turrets with missiles with their bonuses. But usually it is more efficient to decide for one of them especially if you use damage modules or rigs.
For the few ships that can do I have found few good uses for it.
Fit a gun boat as you normally would with light misses or rockets. Use the missiles for frigs while the guns take out the bigger ships is about all. Maybe if you have the power grid for it it could be used for just a bit more pvp dps.