@donavabdear didn't we go over this already :-)
MTM of WMTMW are meant to be listened to on-axis at tweeter height or whatever the tweeter height is based on the total speaker angle. At that height, there will be no phasing issues (assuming I know what you mean). The sound from the two mids-woofers at all frequencies will get to the listener at the same time. The crossover is designed as such that those frequencies all arrive at that same time. This has an advantage over a flat-front MT where the ideal response is not perpendicular to the face but typically tilted down. That can be fixed by tilting the speaker up, setting the tweeter back, or electronically. It can also be fixed with a coaxial driver. I think that is the real advantage of a coaxial driver, consistent dispersion.
The problem with MTM is the vertical directivity is narrow making the listening height more critical. I have not given a lot of though, but the wide spaced woofers in a WMTMW should provide some line source effect and reduce the floor/ceiling mode which is good as those are usually the least treated.
I personally am not a big fan of MTM, and they really are not in favor. We know enough that they do not make much sense any more. Audio Science Review probably inadvertently has given Genelec a lot of press in the consumer market. They have released a great product obviously, but that does not mean other great products not as visibile with similar design goals don't exist. As they are now going after the consumer market, it may influence that segment of the market more than anywhere. The WMTMW is an "audiophile" thing. It does not have to make sense.