They are the ultimate in support and they deal almost no damage themselves. They have more hit points than normal, but can only be healed by a Packmaster. The Rat Ogre boasts 34k health for example.
The Rat Ogre is the tank's tank. Takes it like a man and hits like a girl. He boasts incredible amounts of CC. He's got everything from the AoE stagger, a massive punt that ignores immunities and the ability to hurl a group/warband member out of (or into) harm's way. Put this guy in your anvil group for the ultimate in controlling the flow of the battlefield.
The Packmaster, in addition to being the only thing capable of healing Skaven players, also boasts a nasty plague rat swarm. At one point I had a 50% debuff to outgoing damage and healing from that thing. He can also Pounce to the nearest Rat Ogre who can then toss him further out of trouble if needed. He also has a curious group buff...
The Gutter Runner's signature move in the open is his snare + movement barb. Got a bomb group? Can't have those poor saps getting out of range of your bombers can you? He can also leap out of (or into) trouble. He can also sabotage a siege engine, making it useless until it's cleansed.
The Warlock Engineer is the energizer bunny. He make your group keep going and going and going. He can put an AoE healing/magic power buff or an AoE AP regen buff. He also can cleanse a war engine that a Gutter Runner sabotages. He also has a flamethrower.
None of these classes are very good on their own. Put them with a coordinated warband, and they are deadly. Hopefully the complete lack of damage output discourages the uncoordinated from even wanting to play them. It would be a shame to waste their awesome potential with people afk in the keep.