Beast Handlers are a vital component of the Skorne war machine. They are used to train and condition the beasts Skorne use in battle. On the tabletop, they are a necessary part of your army- healing, enhancing, and just making those warbeasts easier to handle for your warlocks.
For the most part, their stats aren't the reason you take these guys. They have SPD 6, which is perfect for keeping up with the Skorne beasts. Their DEF and ARM are very low, which means the unit has almost no survivability, but since you'll be tagging along with your beasts you can use them to provide screening for these guys. CMD 8 is a little concerning as well, since they are very fragile. It's not low enough to be in danger of constantly failing, but low enough to be worth noting. Finally, these guys are cheap. 3 points for a full unit (of 6!). That's a tremendous value!
Very low P+S, but it does have reach, and with Anatomical Precision it can still kill single-wound infantry. The Barbed Whip also has the Inflict Pain rule, allowing the handlers to place or remove a fury point on the target warbeast. That can be very handy for either starving your opponent of fury... or overloading his beasts with it.
Of course, if your beast handlers are in combat, you're probably doing something wrong. Their attacks are really a last resort.
Anatomical Precision: Great for killing single-wound infantry, assuming you hit. Since these guys aren't likely to break ARM on their targets, this gives them the potential to chip away a point of damage apiece. Of course... they still shouldn't be in combat.
Beast Manipulation: The entire reason to take this unit. Each member of the unit can perform one of these, and each beast can be effected by one each turn.
- Condition: A friendly beast can have any number of fury put on it, or taken off it. This ability is key to keeping your beasts running efficiently. You can easily drop fury off of beasts to make it easier for your warlock to stop Frenzy, or even to give the beast some room to use Fury for its abilities on the opponent's turn (useful for the Cyclops Brute, for instance). I can't see much use for putting fury ON a friendly beast, but I've had to do it because I'm careless, so it's not wholly useless.
- Enrage: Another help on beast efficiency, it gives a bonus to STR as well as forces the beast to charge, slam, or trample on their next activation for free (which basically saves the beast another Fury). Pretty fantastic on some beasts once they get up close the opponent.
- Medicate: Heals a beast. Fairly useful throughout the game, both to keep your beasts alive after rounds of shooting, or to fix a critical aspect before charging in to unleash a beating.
Obviously, which of these is used depends wholly on circumstances. All are useful though, and will get plenty of use during the game.
Kind of a strange thing here, since the Beast Handlers are there solely to make your beasts better. That means they have very simple rules for synergies with beasts: They make them better!
Master Tormentor Morghoul: No real synergies. They do make Abuse better though- +4 STR is downright nasty. They can also heal the damage Morghoul's abilities cause.
Titan Gladiator: Enrage makes this thing scary. Add on another one of the STR boosts, and the Gladiator can wreck Khador heavies. Spirit and Mind are the most likely to be destroyed aspects, and the Handlers can keep them healthy.
Cyclops Savage: I find the most useful beast manipulation for the Cyclops is Medicate. Extra strength is nice, but P+S 15 isn't terribly scary. Healing these guys is always nice though, since they are handy to keep around.
They can be taken in all theme forces aside from Void Seer Mordikaar's and Supreme Aptimus Zaal's.
Master Tormentor Morghoul's theme list requires two units of them for one of the tiers, which causes heavy warbeasts to be cheaper.
These guys are absolutely worth taking in nearly every Skorne army. They make your beasts easier to run, and that makes the beasts better overall. They are also so inexpensive that it isn't a point sink to include them. They are extremely fragile, and will certainly draw the attention of your opponent, but that's just another way to help the beasts out, I would think. 100% take them in every army.