Material projectiles are obviously far more effective against armored targets than a laser/energy based weapon. The thing is that lasers are extremely wasteful in their energy usage. If a laser impacts a solid surface the top layer absorbs all the energy until it vaporizes and then the next layer does the same. Lasers consume a tremendous amount of energy (we're talking about multi-megawatt power supplies) and most of it is wasted vaporizing the metal over a typically small area (unless you using a beam cannon). Thus you need a constant stream of energy to burn through the armor to get inside.
A solid projectile would retain its kinetic energy in itself does not immediately transfer all energy to it's target, but at the instant the armor loses structural integrity it breaks through and moves to the next layer, not wasting time to melt and vaporize the metal and plenty of kinetic energy left to cut through.
So here's how it goes. Your ship gets blasted with a laser. The top few inches boil away to space, but you're still safe. Then the enemy starts using very large, extremely dense slugs and accelerating them to incredibly high speeds (using magnetic acceleration or something) using about the same amount of joules as the laser did and BAM!!! The slug rips through your armor like paper if not flattening your ship completely.
The problem with this is that the projectiles are what they are. Solid hunks of mass so you'd have to haul around huge amounts of mass through space which slow you down and take up precious space aboard your ship.
Speaking of lasers anyway, not too many people know that the word "laser" is not really a word but the acronym L.A.S.E.R. standing for "Light Amplification by Stimulated Emission of Radiation". Just a useless fact.
