Another advantage over the ballistic reduced-bombs is that the bombs cannot maneuver. When you launch the er... missile, with the lack of propulsion, the ship has to STOP MOVING or you have to lead it and it NOT CHANGE DIRECTION while it speeds (still slower than a beam) to the target. Before you point out that beams can maneuver either, the slash beam does, and the beam is direct enough that the enemy ship is just about immobile by comparison. Along with the aforementioned cost, range (The Colossus opens up at the Sathanas at nearly 10 klicks in that one campaign mission IIRC), and logistics, the beam looks pretty attractive.
Lets take this into the real world again

:
In reality, (and here I am talking about space combat, lol) a missile launched from a tube would accelerate as it left the tube, but then, as it goes ballistic, it can no longer maneuver, no longer speed up, and will only detonate if it hits a target (unless you install a proximity fuse, which would be hella expensive, might as well build a fighter). While the missile is ballistic, it can be easily intercepted by ANYTHING. even another bomb, if they have time to line it up right.
The beam
would hit. Computers aim those things, not people prone to error. When it hits, it goes through whatever shields the ship has, and even if they miss, a slash beam will track back onto the target. For all practical intents and purposes, the bomber delivered self-propelled warhead is more cost-effective, has a higher hit probability and is less likely to be shot down. Furthermore, the beam is superior over the ship-based warhead because of: range, ammo, secondary explosions if said ammo is hit, and speed. The way to negate the lack of maneuverability is to install drives, which is exactly what bombers launch.