Robot in disguise
Another thing is that, if your opponent is a manned spacecraft, all you would have to do is poke a hole in it and depressurization will take care of the rest. Now, though lasers can be powerful, they would have to be focused on a single spot for a long period of time, and if the enemy moves, your laser would have to move at the same rate in the same direction in order to keep it's target. Not easy to do. Compared to that, mass propelled weapon could be led ahead of time, fired and then the opponent could attempt to maneuver out of the way of the incoming object before it impacts the hull. With the fact that currently, mere flecks of paint are causing severe damage to spacecraft, (source: http://www.nasa.gov/mission_pages/station/news/orbital_debris.html) items not intended to do damage in the first place, imagine the damage an object which was hurtled into space with the intention of causing severe damage to spacecraft would be capable of.
So, all I have to do to destroy the ship is to puncture it with a 9mm bullet?
But what if they have deflector shielding? Remember, it's the distant future, and I don't think Master Chief could easily take out an enemy fighter ship with just a shotgun.