As Swampert recently educated me in Truant (And did a damn good job at that!), I can answer this with saying that it compensates and balances Slaking out, seeing as it has a massive Base 160 Attack.
If you space it out between the hindering of Truant, that's more like a 80 Base Attack, seeing as you're only attack half as often as the average Pokemon.
Just for balance reasons Bro. Imagine fighting Norman in Pokemon Ruby/Sapphire; fighting against two Slaking without Truant, both able to (mathematically) hit harder than GROUDON. Yeah, I don't want to even dream about it either...shivers in fear