Ah! Okay, this is how the Slavers worked in my scenarios:
I just realised that I upped their Strength from the usual 1 - 2 of the Orcs because I envisioned them to be like the Tippoo Sultan´s jettis in Cornwell´s book Sharpe´s Tiger – strongmen and fakirs who had spent their lives building up an impressive physique, who entertained the Sultan by bending iron bars and executed prisoners by driving nails into their skulls with their bare hands. Ogre Strength lies between 3 (Heavy Infantry) and 4 (Guard and Deathseekers). Personally I think it´s conceivable for an Orc "bodybuilder" to reach a tone similar to a weak Ogre. But I´m fine with the suggestion to cap their Strength at 2 – it won´t make a lasting difference to the style of the unit, after all.
The hooked spears are intended to catch runaway slaves. They are of little use except against panicked models trying to disengage and flee. Think of them as a "story element" – it´s there to add flavour to the model but will rarely ever crop up in play. We had a few scenarios – a slave revolt in a palace, Firstborn ambushing a slave train to free their brethren, a civilian hunt – where the spears actually came into their own. (In those scenarios, a "killed" model was actually supposed to be subdued, so there was no need for additional rules.)
As for tripping, that part is already amply played by the Nets. I don´t feel that there is need to duplicate or add to the Net´s effects by introducing a second way to hamper an enemy model. If the Slavers´ Strength of 3 stays in place, maybe it would be an option to lower the spears´ damage to 8 or even 7. They are slave-catchers, after all, not battle spears.