Why not use mains as centers

>Is there a reason I should'nt use two good bookshelfs or smaller towers for my center if I have two center outputs and available amp channel?

You don't want to use two because one will have more uniform power response and off-axis behavior than two.

I use a full-range main speaker as a center channel....and it rocks. I use an infinity Kappa 9 laid horizontally as my center channel.

Ouch! If you lay that on its side, you are encumbering all (and I mean all!) the comb filtering and irregularity of any horizontal center and, probably, compromising the otherwise ideal match to your other Kappa 9s.

You are a good example of the market for which speaker manufacturers make horizontal center speakers.

Kal

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Kal, technically you may be correct, however,in my system it sounds outstanding. I've tried other speakers designed specifically for center channel duty, and they just didn't measure up. Hearing is believing.

I have two Infinity Kappa Video Center speakers in my storage unit that I need to sell....they just didn't fill my room with enough sound. The size of my room pretty much voids all of the conventional rules that apply to center channels. When the makers of home theater make center channel speakers, I don't think they were considering that the room would be 70 feet long and 22 feet wide with a 22 ft ceiling height. After much experimentation, I found a solution for me. In a conventional sized room, speakers designed specifically for center channels might be a better option...but not in my case.
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I have no problem with your satisfaction but you have traded a dynamically inadequate solution (dedicated center) for a spatially compromised one. If that is optimum, fine.

There are center speakers that will do fine in a large room, such as the B&W HTM1D or the JBL Synthesis units. Nonetheless, I would be surprised if you can achieve a really good sound field throughout with any standard home audio equipment.