Truant: Eh, Slaking would be ridiculous if he didn't have it.
Ring Target: Trick + Ring Target = Profit.
IVs: yep. Truant: doesen't make it any less horrible. Ring Target: Never used trick.
Truant is a pretty terrible ability, but its not like it's widespread or anything. Slaking is a gimmick pokemon, and Truant is his gimmick.
Now Slow Start, that's a travesty. Kyurem-B and Kyurem-W have better stats than Regigigas, yet they don't have such restraints. Why does the mighty Regigigas have to be held back by such a terrible ability? He's supposed to be a legendary, not a joke.