The rear center is used just like one of the rear surrounds in that background music and sounds are primarily what it will play. It's purpose is to smooth the transition between the rear surrounds so that there are no seams or bumps in the sound as it moves from speaker to speaker (think of a helicopter fly by or something). Because of this, the rear speaker should be a diffuse surround speaker just like the other surrounds. A center channel speaker is much more directional so, while it would work as a rear center, it's not the ideal solution.
However, because you have the Quintet system, the surrounds and mains you have already aren't really tuned to their precise jobs. So, since the speakers you have already aren't technically perfect for their jobs, there's no reason you couldn't use a front center speaker as your rear center assuming you're happy with the way your system sounds now. It sure won't hurt you, in other words.
I hope that helps.