I would like to clarify, if I may, that I consider a leader Pokémon not one that I have in the 1-spot all the time, but one I go-to if in a sticky situation. My Pokémon line-up is always shifting. The first Pokémon will always be the lowest level and the six-spot Pokémon will always be the strongest.
I agree with you. My Serperior and Samurott are not always in the first spot in my party. But I have a tendency to use them more than my other pokemon because they are usually more reliable in battles.