Center channels: why only horizontal arrays?

If you cannot get decent mid and tweeter alignment between center and mains the I'd recommend you go phantom for a small audience of one or two. More trouble than it is worth in many cases but you'll definitely need one if you have a crowd over with seats way outside the sweetspot.

There are some good stand mount speakers that are designed to do vertical or horizintal. Like the now discontinued Wilson Audio Cubs. In better times, I owned 3 Cubs: 2 on stands for left and right fronts and the third sitting horizontally on top of my old 52" rear projection TV.

when i used a rptv, i used the horizontal, but after moving to the front projection, i have exclusively used a full range tower speaker. I have used it below my screen when my room allowed and currently use it behind my screen. I believe most of the early designs are based on sitting on a traditional tv or RPTV as it was difficult to figure out how to put that big tower speaker or even small monitor around the center without taking away from the aestitics of the "theater" effect.

03-09-09: Dcrugby...
i'm getting a funny image in my head, though, with the way you've got "aesthetics" spelled there, Pmcneil - i'm seeing a bunch of buddhist monks watching movies on a nice home theater system, but it has a horizontal center channel because they've renounced the luxury and materialism of a vertical center channel...
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Reminded me of this...

"Monastery Life"

A young monk arrives at the monastery. He is assigned to helping the other monks in copying the old canons and laws of the church by hand.

He notices, however, that all of the monks are copying from copies, not from the original manuscript. So, the new monk goes to the head abbot to question this, pointing out that if someone made even a small error in the first copy, it would never be picked up! In fact, that error would be continued in all of the subsequent copies.
The head monk, says, 'We have been copying from the copies for centuries, but you make a good point, my son.'

He goes down into the dark caves underneath the monastery
where the original manuscripts are held as archives in a locked
vault that hasn't been opened for hundreds of years. Hours go
by and nobody sees the old abbot .

So, the young monk gets worried and goes down to look for him. He sees him banging his head against the wall and wailing.
We missed theR!
We missed theR!
We missed theR!'
His forehead is all bloody and bruised and he is crying
uncontrollably. The young monk asks the old abbot, 'What's wrong, father?'
With A choking voice, the old abbot replies,

'The word was...

CELEBRATE!!!'

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.