Seekers are a good counter to small ships as long as they don't have PD. They pack just a bit more damage than a large fixed cannon into a small or medium hardpoint, at the expense of being extremely ammo-starved. Getting hit by that pretty much wrecks small ships once their shields are down (and small ships have puny shields that can easily be dropped by a gimbal or turret laser), and seekers rarely miss. Main drawback is the smallest ammo pool in the game, though you can offset that if you happen to have a large stockpile of iron, nickel, carbon, and sulfur. Great for swatting Eagles, as you'll usually only use one or two missiles per kill (the laser you used to pop the shield can easily finish them off from there), but the tiny ammo reserves make them inadequate for large targets or extended fights.
Dumbfire missiles are essentially fixed cannons, and you use them pretty much the same way. Like seekers, they do the same damage regardless of slot size, so you can fit the damage of a large cannon into a small or medium slot. Dumbfires have more ammo than seekers but less than cannons, and have a slightly slower projectile velocity (IIRC it's something like 750 m/s while a large cannon is around 800). And of course, neither missile type has anywhere near the ammo endurance of a multicannon.
A big drawback that applies to both types of missiles is that they have very low internal module damage, and a low chance of actually inflicting that damage. So while they fit a lot of damage into a small/medium slot, they're not good for sniping powerplants. For seekers this doesn't matter much since you'll be using them to swat targets that pop long before any of their modules do anyway, but it's definitely something to keep in mind if substituting dumbfire missiles for cannons: if you make a habit of sniping powerplants, missiles will hurt your ability to do that.
If you just want to break hull and don't mind the ammo problems though, dumbfire missiles do allow a very high alpha strike when they hit because they essentially let you put large weapons in your small/medium hardpoints, while putting lasers in the large hardpoints.