Although the cost of the beckoners (5) is less than half a twin, they are rare 2, so putting 3 of them in your list is not an option.
Beckoners are extremely useful in a jacob crew (borderline auto necessary). Jacobs crew works off of opponents models having been given the brilliant characteristic. Beckoners are good at moving enemies around, and giving out the brilliance at 12" range. Jacobs crew gains some good bonuses against models with brilliance, so getting it on them, and then focus firing the affected model will be the first line strategy. They have some nice defenses, and their ranged weapon uses their wp (6) instead of cb, and also ignores cover.
The damage is indeed low 1/3/4, but its really about bringing the brilliance, so I am totally cool with that damage spread. They have a mid game ability which I believe will be important, once models start to engage, called the party never ends. Brilliance is a state that ends during the resolve effects step every turn, and with it, ends all the bonuses. The party never ends gives the beckoners a 6" bubble that if models are within, they don't loose the brilliant characteristic at the end of the turn. I think this will be clutch, as most models need to be damaged to get brillianced, and this would take some extra actions without the nice bonuses.
In a non Jacob crew, they are still good movement pieces. When they sucesfully beckon, they get to move also, so they can not only reposition the enemy, they can get into (or out of) other models way, or better position themselves to take out wounded enemies. Their attack score of 6 isnt shabby, and a moderate damage of 3 is enough to finish off an injured critter.
They have one other nice ability. I think in a jacob crew, they will be too busy dishing out brilliance to use it often, but they have a spell called sales pitch. this spell takes away a general action point for every mask in the casting (spell requires at least one, which the beckoner brings with). So if your opponent brings up that big nasty melee critter to beat on your guys, this spell can help you deal with it. Especially useful against models with flurry or 2 cost spells/actions, or charges.