First of all, combat challenge marks any creature the fighter (weaponmaster) attacks--hit or miss, at range or not! That can be very useful, tactically.
Furthermore, a defender aura doesn't do very much if the defender gets pushed around with forced movement. The fighter who marks an enemy might not be able to punish that enemy if he gets moved out of position, but that marked enemy will still have -2 to hit other targets. That's a big deal.
In addition, the fighter (weaponmaster) has combat superiority--which is tremendous! When an enemy provokes an opportunity attack, that opportunity attack is with a bonus to hit and, if successful, stops that enemy cold in their tracks. A fighter is very hard to get past--or run away from!