In addition to what TLS Guy said about tilted axes above, MTM speakers provide the obvious advantage of using two mid-woofers instead of one.
You get greater volume while avoiding greater distortion due to the displacement required of a single driver. Two mid-woofers will not provide deeper bass than one similar driver will, but they will be louder and cleaner sounding.
An MTM also allows using smaller diameter mid-woofers that perform well at higher frequencies than a single larger diameter woofer. This allows crossing over to a tweeter at higher frequency, and provides better dispersion near the crossover frequency.
If you are talking single drivers, yes. However, we are now getting into the power handling arena, and the issue I have brought up for years, about where the power actually is. Everyone thinks its in the sub range when it isn't. It is the upper, mid bass, and the midrange below around 3.5K. I am astounded as to how many professional designers get this wrong. The power is especially required below the transition frequency. Of course the two drivers in MTM in either mid bass, or as midrange drivers, really helps the power handling in this range.
Unless it is a small bookshelf, then two drivers in this range are required. However a 2.5 way can be a good design, and there are plenty of them. However, I do think a well designed MTM does have some advantages, unless you are prone to listening to your speakers from the standing position, or want a downward tilt for say a surround speaker say.
I think many commercial three way systems are a problem. There are precious few midranges that really have adequate power handling for the job, very few. So in a three way, two mids in MTM, I think should be the rule. I recognize that the commercial designer has a bean counter at his elbow. He told the speaker can not cost more the x.