Motivation

The main purpose behind competitive activities in the classroom is to generate an environment where everyone is completely involved.
In so saying, naturally, it is imperative that teams are equal.
However, incorporating groups with unique qualities into the competition will give the individual team members additional incentive to achieve their highest level of scholastic performance. If you place a random group of people in a group, they will have a marginal level of identity within that group, i.e.. Group 1, Group2, etc.
If you place them in groups of people with similar characteristics, you will create a higher level of identity of the group. Specifically, if you create groups (such as Senior girls, plus a couple additional people to enlarge the group, senior guys, junior guys, and junior girls) with specific "identities," in this case, mainly grade, it will inspire a more competitive spirit. Instead of merely hoping that one's group of random people (whom one does love being on a team with) will win, one will be rooting for Senior Guys/Senior Girls, etc. This will motivate each group to win.
This motivation will lead to higher involvement and personal achievement.
In the end, it is not whether one group wins or loses. It is not whether one group has two points more at the end of the game. What truly matters is that the students are inspired through the class to delve deeper into literature and truly engage in class. That is what I think this particular method of motivational competition will achieve.