For Singles, Slaking is a generally a Hit-and-Run Pokemon - it carries a Choice Band along with moves like Superpower, Hammer Arm and the like. Since Truant lets you switch, it gives you this kind of freedom.
For Doubles, Slaking is a lot more viable. A good partner is anything with Skill Swap, but for the purpose of this exercise, I will work with Reuniclus. First turn, Skill Swap the Slaking to give it Magic Guard (which also becomes beneficial to it, in that the Slaking could carry the Flame Orb. If you then carry Facade, that is extreme damage with no recoil. However, you are still crippled by the burn for your other moves). Reuniclus can then Skill Swap on the following turn to give an opposing Pokemon Truant. However, this strategy is a bit risky.