Playing with more of a type in your team should provide small buffs to make type-focusing make more sense, outside of "I just like them," or "They're easier to train when I'm already experienced in their type," which I think are the canon reasons.
Think 5% more to all stats of any Pokemon with a shared type. Pure types give an extra 5%.
So pure normal pure type team would each have something like 60% bonus to all stats, but you'd be able to prepare with six fighting-types.
