Why not use mains as centers

Using a main speaker as a center is ideal and usually superior to using most dedicated center speakers. However, you should use only one, not two, or the radiation/interference issues common to horizontal speaker will be encumbered.

Use one. Save the other for a spare.

Kal

I use two bookshelf speakers on their sides, on top of the sub, tweeters in, and making a V angle out of about 30 degrees. Sandbox on top. No interference issues apparent in movies. I play music elsewhere.

I would expect a good specialized centre channel speaker to be far superior than a third main. The centre channel in an HT setup is primarily for voices. Voices are in the midrange. So the design of the centre speaker can focus on midrange reproduction. There is less need to design for bass. The left and right mains, on the other hand, need better bass capability. So their design will be obviously different. Specialization of function is, I think, the primary reason why centre speaker design differ from that of other speakers. Also, HT speaker design will, or may, at the manufacturer's option, have to accommodate the design requirements needed for THX certification.

You can't assume that what's desired in a speaker for music reproduction in a 2 channel system is what's needed or desired in an HT set-up. That's why a lot of people have dedicated systems for both 2-channel and HT use. That's why you see so many posts on this forum from people about how to deal with the compromises when they want to use a single system for both purposes.

Sorry, Markphd. While there are a few well-designed center speakers, the vast majority are specialized to accommodate their positioning, not their performance. In specific, horizontal speaker arrays are fundamentally flawed and that is why you see so few of them used for stereo or main channel speakers.

As I said, there are exceptions but they are few and far between.

Also, Samujohn, it is fine that you are happy with your paired horizontal speakers but that does not mean they are not compromising imaging due to radiation interference.

Kal

I helped a buddy set up a great home theater, with a lot of room, and we actually used a pair of Magnepan MMG's because he had the space and an extra pair, man they sound great. We had no interference problems, and actually just split the signal with a good DH Labs Y-splitter from the processor. It has worked great. It's a bad-a__ theater. All Maggies: IIIa up front (rebuilt at Magnepan), MC-1's for the rears, the pair of MMG's for the center and a single Velodyne DD-15 in a fairly large room. Very nice for music, great for theater. Anthem D-1 with an Innersound ESL running the fronts and a Parasound 2205 running all the other speakers, it's a really bumpin' home theater, turned out great.