"On your turn, you must engage the target you challenged or challenge a different target." On your turn, engage OR pick a new target. That is the RAW. This means that if you want to, you can switch back and forth between multiple targets and never engage. If this offends your concept of a paladin, do not use this tactic, but that isn't relevant to the rules.
The issue seen at D&D Experience that prompted a change to DC was the exploit of fully running away and the DC remaining active. In the case of twisted tunnels with a double move, that meant the enemy would never be able to safely attack ANY target. The paladin's best tactic was to run from the final dragon in the Scalegloom module. Now, you must engage an enemy to keep the DC active on that creature.
The role of the paladin is defender, and that means keeping the enemies occupied. In the standard 5 on 5 of encounters, what use is a divine challenge that can't effectively change for role that is intended to tie up enemies? You tie up one, the rest gang the wizard. That does violate the concept of the class.
In the example we started with, the ant eating your friend, you can give your friend a chance by DC the ant. This is only temporary if you can't get to the ant on your friend but it is better than nothing. Remember your role as a paladin: defend. You are there to soak the agro. If you can't engage the ant attacking your friend, you either challenge another creature, (leaving your friend out to dry) or lose DC for a turn (leaving your friend out to dry).
For those of you who consider this an abuse of the rules, consider a pair of elven paladins with bows, firing from the trees. Engage explicitly includes "attack" and not "melee attack" in it's definition in Divine Challenge. This means that I cause a melee character damage every round if he doesn't ineffectually attack me or waste his turn. This is perfectly legal by the rules even as they have been interpreted by most here.
In short, the rules say, turn by turn, engage or pick a new target. There is no requirement in the ability to engage every round.
The issue seen at D&D Experience that prompted a change to DC was the exploit of fully running away and the DC remaining active. In the case of twisted tunnels with a double move, that meant the enemy would never be able to safely attack ANY target. The paladin's best tactic was to run from the final dragon in the Scalegloom module. Now, you must engage an enemy to keep the DC active on that creature.
The role of the paladin is defender, and that means keeping the enemies occupied. In the standard 5 on 5 of encounters, what use is a divine challenge that can't effectively change for role that is intended to tie up enemies? You tie up one, the rest gang the wizard. That does violate the concept of the class.
In the example we started with, the ant eating your friend, you can give your friend a chance by DC the ant. This is only temporary if you can't get to the ant on your friend but it is better than nothing. Remember your role as a paladin: defend. You are there to soak the agro. If you can't engage the ant attacking your friend, you either challenge another creature, (leaving your friend out to dry) or lose DC for a turn (leaving your friend out to dry).
For those of you who consider this an abuse of the rules, consider a pair of elven paladins with bows, firing from the trees. Engage explicitly includes "attack" and not "melee attack" in it's definition in Divine Challenge. This means that I cause a melee character damage every round if he doesn't ineffectually attack me or waste his turn. This is perfectly legal by the rules even as they have been interpreted by most here.
In short, the rules say, turn by turn, engage or pick a new target. There is no requirement in the ability to engage every round.