RTR: I always thought it was better to wound than kill.
Modern infantry combat (for which the weapons and ammo in question are designed) happens at rather short distances - below 300 yards and often below 50 yards.
The 5.56 bullet that does not desintregrate leaves a neat clean hole which usually closes due to natural elasticity of the tissue.
Unless a vital nerve center or joint is hit, even a mortal wound may not be severe enough to disable the victim quickly and prevent him from hurting you.
If the enemy is under influence of alcohol, drugs or even under adrenaline rush, he/she may not even notice being hit for a considerable (in combat) time.
There is an instructive description of using AP ammo from a short-barreled variant of M16 in "The Black Hawk Down" by Mark Bowden. Somalis hit with multiple rounds just ignored them for a while.
miko