Precisely....we realy don't need a light type...or any new type for that matter. If they introduce a new type; that said "new" type would be put in to deminish another type from being to powerful. Currently we don't have that problem.
What self-assigned notion compels you to speak on Game Freak's behalf? Type balancing is not the only reason Game Freak might introduce a new type. At this point, a light type is aesthetically warranted, especially if they want to have a good versus evil theme with Xerneas and Yveltal (and I'm guessing they do, since "evil" is easily spotted in Yveltal's name).
Plus, a light type should have been added in when a dark type was. It's time Game Freak filled that gaping hole.