I can't say I've used Draw Back much, but I freakin' love Swap. Swap Stahl is the perfect companion for a unit like Olwen or Cordelia.
As for Threaten skills, I've found a couple of uses for them:
Threaten Speed can be really useful in enabling fast units to double anyone up to their own speed bracket, all you need to do is position your unit (in my case Cordelia) just outside the range of a melee range enemy and they'll move their maximum movement and end their turn 2 spaces away from you. Then you start your next turn, their Spd drops by 5, and you move in for the kill. It takes some finicky positioning, which means it doesn't always work in the heat of battle, but after a while it becomes second nature when lining up your units to take the initial charge.
Threaten Defence is fantastic on armoured units, as it is essentially Death Blow. Any unit that an armoured unit can attack (without being helped by Reposition) must be within two spaces of the armoured unit at the beginning of your turn, otherwise you won't reach them. Therefore an armoured unit with Threaten Def 3 effectively has 5 more points of Atk, and what's more it also works off your phase once it's initially applied. This is the reason Eckesachs works so well on Zephiel, it's almost like a flat 20 might weapon. Threaten Defence can also be used in a similar manner to Threaten Spd, but for brave weapon wielders. A free 10 points of damage just by lining your units up correctly is nothing to sneeze at.
Threaten Resistance can be used in much the same manner as Threaten Defence on brave weapon users, but on Dire Thunder users instead. Again, it's 10 free damage (often on top of 12 free damage from Death Blow 3).
There's also the infamous cavalry buffing skills, which are amazing at the moment, while their flier and armour counterparts are somewhat less so. If your unit is unlikely to benefit from any of that though, then there's nothing wrong with slapping a Hone skill in their C slot, a Rally skill in their support slot, and turning them into a dual purpose unit.
Edit:
@Chase I don't think he was talking about Azura, as either would require her ditch Sing.