I believe the concept behind the hague and geneva were to set some rules to war that "civilized" nations would abide to. The point of war not being total annihilation of the enemy army and enslavement of the people, but to render the enemy so weakened that they have no choice but to surrender.
Before the accords it was common practice to kill all the enemies surviving army, enslave the population, rape the women, sack the cities, crucify, impale, burn alive, and all the other inhuman things we now regard as barbaric.
If FMJ isn't effective enough than why not use hollowpoints? Well why stop at that? Why not poison every bullet so even a minor flesh wound is lethal? Bullets are ineffective way of doing things to, nerve gas is much more effective, bio even more so. And nothing can beat a nuke, except maybe an asteroid impact using remotely guided boosters to nudge it into the proper orbit to impact the enemies country.
Where do you draw the line at how "effective" (destructive) a weapon is before it becomes too "effective"? This is the whole point behind the accords. To limit the scope of warfare because our ability to build apocolyptic (sp?) weapons outstrips our good sense not to use them.