If the primary purpose of Melee Engagement was to enable characters to "hold the line", why should it be a factor when they're the ones charging forward or moving around the battlefield? Why should it be a factor when there are no "lines" to speak of at all?
Make it a modal ability that you need to toggle (including a short activation/deactivation time) which plants you in place, like deploying a Siege Tank in Starcraft:
This would prevent so many engagement exploits and it would make things a lot easier to understand. The button for activating it could take the place of the infamous "Guard" button from the Infinity Engine games that nobody used.