I am not sure I understand the objective of this BG.
Play ashran until you get sick of being there then afk out without deserter penalty.
Need to do an event or kill boss to finish your quest . You can afk out after that with no penalty or stay in the game to honor farm. It never ends .
Does killing a rare count as an event?
No. Killing rare does not count as an event, but it might give you some toys or scrolls that would help you push the enemy back.
There is no definite objective, and killing enemy boss is not the only option. There are groups only keen to do events for honor points, or roaming the map just to troll the other side. This free-form nature is perhaps the biggest weakness and the strongest point, depend on who you ask. Either way, Ashran is different and can be a lot of fun 