Not all clerics are built the same some focus more on spells, other more on combat. Healing also includes remove condition. The point is there is no one right way to play any particular class. The combat focused cleric could also be memorizing long lasting buffs or other situational spells. I also never said the cleric was memorizing healing spells.
If the fighter is already hitting 90% of the time and the cleric is hitting 70% of the time giving the more powerful weapon to the cleric will have a larger impact on the combat. If the weapon gives an additional +3 to hit that puts the fighter at 95% accuracy and the cleric at 85% accuracy. The extra 10% chance to hit is wasted on the fighter, but the cleric can make better use of it. The extra damage from the fighter using the weapon is minimal compared to the extra hit from the cleric.
Having to buy two magic weapons also gimps two weapon fighting. The ABP also gives the option to split the bonus instead of taking it all on a single weapon.