As an example take this scenario:
You're a Paladin. You begin your turn by Challenging an enemy, who is standing 5 squares away from another enemy. You must now pretty much go into melee with the marked enemy. The other enemy attacks your friends, and moves several squares or so away, so he is now 9 squares away from the Paladin.
Next round, you are fighting the original mark, because you pretty much have to. Your original mark dies to your Standard Action for whatever reason. Now the other monster has free reign for an entire round even if you Ran after him with your Move action. It wouldn't matter if you used your Mark now even though he is in range after your Move action, because it would immediately end if you weren't adjacent and you cannot make another attack (assume no action points).
Not doing such a great job as a Defender there in my opinion, if you can't even take heat off of your friends from one other monster for one round. And, what's more, if you tried to by Marking the enemy when you couldn't have gotten adjacent anyway, you wouldn't be able to Challenge next round!
So that would be 2 rounds of no Challenge for the Paladin who is running up to the next monster, doing his darndest to Distract the creature and call down his deity's righteous fury, all to help his friends! Apparently his deity doesn't take kindly to being called upon if his servant is not immediately next to a bad guy or swinging at its head...
Anyway, I know examples always end up being stupid (poor tactics and whatnot), but this isn't that uncommon a situation (the only remaining enemy or most threatening to allies is out of a single move action).
I think allowing the Challenge at the end of the turn fits the flavor, it fits the role, it fits player expectations, and I feel fits the Intent of the rule.
You're a Paladin. You begin your turn by Challenging an enemy, who is standing 5 squares away from another enemy. You must now pretty much go into melee with the marked enemy. The other enemy attacks your friends, and moves several squares or so away, so he is now 9 squares away from the Paladin.
Next round, you are fighting the original mark, because you pretty much have to. Your original mark dies to your Standard Action for whatever reason. Now the other monster has free reign for an entire round even if you Ran after him with your Move action. It wouldn't matter if you used your Mark now even though he is in range after your Move action, because it would immediately end if you weren't adjacent and you cannot make another attack (assume no action points).
Not doing such a great job as a Defender there in my opinion, if you can't even take heat off of your friends from one other monster for one round. And, what's more, if you tried to by Marking the enemy when you couldn't have gotten adjacent anyway, you wouldn't be able to Challenge next round!
So that would be 2 rounds of no Challenge for the Paladin who is running up to the next monster, doing his darndest to Distract the creature and call down his deity's righteous fury, all to help his friends! Apparently his deity doesn't take kindly to being called upon if his servant is not immediately next to a bad guy or swinging at its head...
Anyway, I know examples always end up being stupid (poor tactics and whatnot), but this isn't that uncommon a situation (the only remaining enemy or most threatening to allies is out of a single move action).
I think allowing the Challenge at the end of the turn fits the flavor, it fits the role, it fits player expectations, and I feel fits the Intent of the rule.