IMO the biggest advantage to adding a center is for those not sitting right in the sweet spot. If you watch TV alone and always sit in the same position, it wont really do much for you. But most people move around or end up with people sitting a little out of the sweet spot. For those people, the center speaker helps to anchor the dialogue to the screen. The secondary advantage is that it takes some of the load off of your other speakers. Right now all five channels of audio you're receiving are being downmixed to two speakers. Lessening the burden of half of the center channel's content from each of your existing speakers will pull them that much farther out of distortion caused by power compression and excursion.