I too have played with both single and dual speaker center channel arrangements. I use my system for music and movies as well as regular and HD TV. I have it setup for stereo, Quad, 4.1, 5.1, and 7.1.
I like the way a single speaker array works, but I am going to do some more listening with a dual speaker setup as soon as my old Sansui AU-999 is finished being restored and recapped. It has the capability of running L+R mono, as well as reverse stereo.
http://www.sansui.us/images/AU999_00051.jpg It will be a while before my tech (QuadBob - quadbob@prodigy.net ) can get to the 999, as he is very busy with many other projects, including my third Sansui AU-D11 ii, so I'll let you know when everything is setup and working.
But to answer your question, I like the full range sound you get from dual full range center speakers playing a dual mono signal.
Lonster