Truant was originally the Slakoth evolutionary line's signature ability, and considering they're based on real life sloths, that's one reason why Slaking has Truant, and also a cap on Slaking's power.
As for why Durant has it, who knows. However, Durant has other options apart from Truant, like Hustle, which is amazing w/ Hone Claws. Durant also has Entrainment so it can pass Truant to something else, switch out to something with Shadow Tag/ Arena Trap and Protect stall.
For Klutz, all the Pokemon that have it have other options for abilities. Klutz is useless on Audino and Golurk, but for Lopunny it could hold a negative item like Flame Orb or Iron Ball and use Switcheroo on the opponent without the items have a negative effect on itself. Swoobat as well, since it can learn Trick.