More than one person has suggested that the horizontal array of most centers causes suck-out problems off-axis. It seems like a pretty minor deal to me.
OTOH, one little M2 on top of the TV is bound to look funny, don't you think?
And, though I haven't tried it, I'm guessing one M2 would really call attention to itself - pinpointing the location of the sound source.
But don't take my word on it - I haven't tried a vertical center (although I was very tempted when I had 3 M60s on hand!).