The best reason I can give people for using an identical center to there mains is that you never see anyone using 2 different left and right mains presumably because they share a lot of audio information between them so you want them to sound as similar as possible. Since a center seems in most material to share even more information with the L/R mains than those speakers even share between each other why would you want to use a different speaker unless you had to.
uhmm have you heard M&K they are awesome
I had actually considered 3 horizontal centre speakers across the front, but I wasn't able to find any centre speakers that sounded as good as the mains. I'm sure they're out there, but space and $ limitations unfortunately dictate what my options are.
