I believe that ESFP's are the best leaders when they fully mature and can learn to take responsibility for themselves. People say that due to low Ni they don't have a good vision for the future, but when they feel obligated to lead others their Ni really kicks in, as well as their Te.
If the only argument is that an esfp is a better leader when the functions are developed, then you have no argument. Really, any type could be a good leader. You don't really just use a certain function, it plays into the overall personality of a person, its like a blend of colors. So to say someone could just use a function to its fullest extent would just be wrong.