Just started a new game at the beginner level in which my neighbors are slavers. They have no cities or resources in their territory, yet their army is bigger than mine, and they have tanks at the very beginning of the game. To top things off, a two hundred man slaver battalion has held off my 3000 man regiment for several turns in a row, inflicting heavy losses.
While I'm wrestling with the slavers, who have absolutely nothing worth conquering, my other neighbors are now encroaching from two other directions.
In my opinion the slavers need to be nerfed, because this isn't fun at all.
Yeah the slavers can be difficult to defend against if you aren't prepared, especially early. And you're right, there is little to gain from beating them except...defending against them.
I've used a "please all except the one I can deal with" strategy, and it has solved that early push problem. When they asked me to buy slaves, saying yes increases relationship. Keep saying yes until your relations are high enough to play "propose peace" from the minor strategems. That will stop them from constantly pushing into your territory, and makes them easier to manage.
I found that whatever penalty you get in the regime profile or happiness, far outweighs a <10 turn war with slavers. I will keep them happy all game as I conquer other regimes that can actually add to my empire.