People enjoyed campaign battle pre-abyssea for a good reason. It let the individual do his/her own thing in a battle where they were never through a mixture of NPCs and other players. At the same time, it gave halfway decent exp, and took at lot less time commitment than a normal exp or merit party.
I'm guessing this is why the devs made bastion. Bastion hits some of the strong-points of campaign, while giving tons of exp if done correctly. However, theres one problem that holds it back so hard that people stay away from bastion for the most part.
I believe that problem is, how easily the martellos die. After losses the martello has very little health, and can be destroyed in the AoE caused by as little as 1-2 monsters. So from the individual player's standpoint, you need to keep the monsters away from the martello. Many times however, you just can't do this. There are blm npcs hitting the monsters with -aga spells nearly every bastion. At this point, many of the monsters go to the blm npcs which are standing RIGHT NEAR THE MARTELLO. At this point the npcs themselves have lost the battle for you, as in a couple AoE monster abilities, the martello is destroyed.
Note that there are npcs called Clone Wards, designed to be a target for the monsters to hit before they can start focusing on the martello. However, yet again, the npcs can still screw this up, as the martello can be hit by AoE before all the wards are destroyed.
It is very deterring to jump into bastion that may happen only every 3-15+ minutes, and have it end within 20 seconds of the monsters reaching the npcs, followed by the npcs drawing the monsters directly to the martello, and have it AoEd down.
All that would have to improve bastion by a fair amount is to make the martellos immune to any AoE damage. By the time the npcs die, and the players are overwhelmed, you should lose the battle. But losing the battle with all npcs alive, all players scrambling to get the monsters away from the martello is just a bad design.