The primary advantage of blasters is their rate of fire. A sword, for example, swings once every 90 "time units", and this is modified by Speed stat, Armsmaster, and so on. At the end of the game, with high stats/skills and Haste, perhaps your swordsman has reduced it to a cooldown rate of 30, which is as fast as melee weapons are allowed to get. But a blaster starts at 30 and caps at 0, which is quite reachable since Speed/Haste affect blasters normally. (Note that the Haste spell is bugged in MM7 and doesn't work, but Haste potions work normally. If you don't want to patch your game to fix the spell, I encourage you to mix some, they're very useful. Red/orange layered.)
Blasters also deal their own damage type, which nothing in the game resists. (Lategame enemies typically have some resistance even against physical attacks). Accuracy is quite good, too. But it's the obscene rate of fire that breaks them.