I change teams mostly for the battle frontier/competitive battling. Say, use a bug-type team or my trick room team or even my "elite teams" specifically fit for 1v1/2v2/3v3s... it always depends on the situation. But for the most of it, I replace the whole team with different pokemon so i wouldn't get too bored of the game.
But I mainly just bring 3 pokemon + HM slave(s).. so the one or two remaining slots could be for any pokemon i've used before. If i miss them, that is.