Remember it's not only the speaker chosen, but where listeners are placed (as noted above), and where the speaker itself is placed. Even with a "decent" horiz center speaker, if you choose to shove it inside (an extremely resonant/resonant) shelf, and say it's rear ported as well, and then it's also placed far too low, without being angled up, and a coffee table is also blocking its path, well, the odds are that omitting the center is better. I'd sure vote for no center there. Worst case scenario, maybe, but just making a point, there are all sorts of scenarios you see with the CC, I'm sure you've noted.
OTOH, if it's placed at ear level, on top of a cabinet, but also decoupled or isolated from it, you're not limited by a puny size, etc, then I vote for having the center.
So otherwise the issue that seems to be most talked about is off axis response. If you're front and center, I would definitely still go with a horiz MTM (assuming I had to go horiz on a limited budget). Coincidental drivers will not have the same offaxis lobing issue, and the WTMW design have the offaxis lobing to occur at low enough frequencies, where they are simply not offensive to our hearing senses.
