Opinions differ on whether a rear center is necessary. But experiences of many do point to a more immersive surround if there is a rear center. Especially if you have a fairly large room and there are more than two of you that must share the sweet spot. I guess in some situations, with all things equal, you can never go wrong with more chanels and speakers,
Personally, I have no need for one. I neither have the place nor the room size to put one. Also, center channels in a 6.1 receivers are said to simply matrix the center sound in the same way that Dolby prologic did when extracting center sound from stereo materials. Ofcourse there are already ES and EX encoded DVD's out there but the center is still matrixed, not discrete, if not mistaken. But whether matrixed or discrete hardly matters. The center sound is different from either rear channels.
I suggest you ask your dealer to demo a center magnepan for you since he is most eager to sell one, I'm sure. And you're his avid customer. If it's ok with you and you are thrilled with what you hear with a rear center, then consider it sold. IF not, the dealer would retain your patronage by taking it back.
