Personally, I think any 3x3 tower is worth manning, unless possibly it's the only one.
not always. even if there is only one 3x3 tower, it can be worth manning depending on the size of the fort, cause that effects the number of attackers. if there are 140 attackers who may all be shooting at this one tower, then yes, there is no point. but if it is a small fort and there are only 50 attackers shooting at this tower, it may be worth it. all of the occupants will die, but the damage those 9 could inflict and the number of rounds they could hold off the enemy for could be crucial. there are always the offliners or online noobs who advance toward the edge of the tower, where they can't see the defenders in the back opposite side of the tower. after a few attackers have already died, and the noobs have advanced to the edge of the tower, those defenders may only be up against 30 attackers. of course, one or two of the suicidal defenders has prolly already died, but they are still able to inflict damage.
i remember specifically setting this as a strategy. i had to condemn 9 men to commit suicide, which they bravely did, but in doing so we separated the attackers enough to stop them, which was the most important part. when the attackers plan is to mount the walls and towers, to shoot at defenders inside the fort, manning that tower can be crucial. offliner attackers jumped on the walls and in another tower, and got slaughtered by defenders inside the fort. meanwhile, some of their allies were being held up by this one tower, effectively dividing an attack force so that the rest of the defenders could fight smaller groups of attackers rather than the whole group at once.