We all have played that game (usually RPGs) that let you control a group of characters, but not all of them at the same time. So you are forced to left some of them out of the party. Usually the most annoying/weak ones, but sometimes the ones that yes, they are strong, but they aren't as cool as the one you choose.
Another problem is that after leveling up a certain group and getting to know how to use their skills, you get less inclined to use different characters that appear later in the game. You may have already made them stronger than late additions or created some attachment on them.
So, it is an interesting question. What is your criteria when selecting a party?