Center channels: why only horizontal arrays?

Aesthetics. A vertical array will have better off-axis performance although you probably don't have seating far enough out.

A speaker with identical drivers, cross-overs, baffle width, and enclosure volume will provide the best match and not have anomalies where a sound track spreads music across the front 3 speakers.

Definately, matched is best. However, that's not always possible, especially with rare, or very expensive, or huge mains.

I wired two Klipsch RF-7's (modified horn crossovers) together in mono through my processor, flanking my 60" monitor. Each RF-7 gets dedicated amps from my Proceed AMP 5.

They are toed-in slightly and far and away exceed the sound of a single RC-7 on a stand below the screen (good for smaller rooms). It also looks much better.

Dual-centers in horizontal arrays is ideal with a large monitor in my 30 x 30 space. Realistic soundstage, spaciousness, and lots of clear headroom. Much improved in every way.

Rather, the RF-7's are positioned in the usual VERTICAL array flanking the monitor.

There are speaker companies that will not release a center speaker because they feel a horizontal speaker is too much of a comprimise over a vertical speaker. They also do not have enough room to make a proper vertical speaker for HT.

I'd like to see someone like JL Audio design about three sizes of center speakers with room correction. They probably need to be powered, fit flush below various monitors with a special cabinet, and allow for highly variable tuning.