I see. Because Leftovers and the like are near omnipresent in single battles. I don't do single battles as often as the others; I'm also used to how that sort of passive damage/healing is done at Level 50.
And I suppose different strokes for different folks. I dislike the idea of having to have a lead, or at least begin with the same Pokémon all the time, and I like seeing what's on my opponent's team, figure out what they're likely to use, and prepare accordingly. For instance, I have a double battle team on PBR where four of the Pokémon are weak to Ground. I deliberately let people send out their Ground-types at the beginning (or I see what can have Earthquake or Earth Power) and send some unexpected surprises their way.