Jeez... Maybe its just cooler to have big beams and crap. I believe the launching mechanism for the bombs is the bomb itself. So as long as the ship can carry the bombs, it'll work. For any large ship to fire bombs, it has to be at a safe distance. Imagine how the Psamtak would of been if it had to use bombs instead of beams on the Belisarius. At a safe distance, the bombs can be easily shot down and can even do damage to the ship launching the bombs. The thing about bombs is, they're used as a tactical weapon although the Harbinger was said to be used as a planetary bombardment warhead.
So to overcome this difficulty, bombers are developed to deploy the bombs. In addition, multiple wings of bombers can be used for multiple missions where a single destroyer can only be at one place at a time. A destroyer's two primary roles are, to exercise power (meaning to attack and show off your ship) and to carry fighter/bomber wings. Missiles can be evaded by counter-measures and the missile hard points on a destroyer can be easily destroyed.
Overall, beam weaponry may actually be cheaper to deploy then warheads because it doesn't require physical storage space where beams are probably stored as energy thoughout the entire ship. Also, having a bombs and missiles located near the outer hull is another bad issue. Taking out a single missile battery would comparable to setting off a small nuke right on the ship. Beams are less vunerable, cheaper in the long run, and have a much longer range.
