by dougtempel
I'll note that the rule itself is not optional, but the act of voluntarily engaging the enemy is optional. If you never choose to voluntarily engage an enemy, then it's just a matter of semantics.The idea behind optional engagement is that it allows you to engage enemies whose threat level is higher than the current threat level of any player. This removes threat from the staging area, which can make questing much easier.
The other advantage in a multi-player game is that a character that's more equipped to deal with (i.e., defend and attack) the enemy can preempt the enemy from engaging a different player who may not be ready to deal with it.