Actually the 100 yard zero is best for an AR15 used in Home Defense where the threat will most likely be under 100 Yards and mainly at 25yards (100 feet) or less. The problem is in the 75 yard shots with a 50 yard zero. While all close up under 25 feet will have the same error. You do not want to zero at a closer range than 25 yards in any case. Of course if you feel that 99% of your use will be 50 and under the 25 yard zero is best.
The logic is in the height over bore issue and upward trajectory issue. When you go to pop a threat vary close with a 50 yards zero your shots trajectory will be 2" or more off. If you only have a head to shoot at or someone using cover against you that little 2" may matter. If you own Magpuls Art of the Tactical carbine, they go over this issue after they zero at 50yards. All zeros have this issue to some degree. You have to aim under or over [just slightly] when under stress your not going to do that. Many people recommend a 50 yard zero if / when engagments past 50 and out to 250 will be possible. For a strick HD AR15 you need to tighten up that trajectory as much as possible which means using the 100yard zero for the 100 yard threats. The 25 yard zero is the worst for close up work. Here is another view of that chart showing the whole trajectory of the 55gr bullet shot from a 1/7 twist. If you feel common threats will be past 250 yards the 25 yard zero is again better due to the cross trajectory zero of about 375 yards. In any case up close and person shooting is stress full and your aim is going to be very far off, every little inch matters.
EDIT- After reading this several times I feel I didnt make myself clear. there is nothing wrong with any zero as long as you know what the trajectory is and under stress you will account for that error. The 100 yard zero is a better close up zero only because you only have to "adjust" your shots one way. The 50 yard zero will have both over and under trajectories depending on threat distance. So under stress your training of only adjusting one way will be quicker.