I know it has been brought up here many times that the 2-way MTM horizontal center is a poor choice, which in that specific scenario running Phantom
could be preferable.
Personally, I think centers are more necessary and yes, they should match as closely as possible the voicing (at least) of the Mains. In my posts, I usually refer to the Front 3, not just Mains.
Now, I do agree that the approach to talking about the Front Stage is important, and that the options for how to set up your Front 3 properly need more focus. I like the idea of either horizontal 3-way Centers with a W/TM/W alignment, or matching Tower or Standmount of the set one is buying. Likewise, if one is keen enough to audition critically, finding similarly voiced speakers that may not be matching is perfectly fine, too.
I also agree that in many instances, due to the unfortunate focus on size constraints and shoehorning a Center where it doesn't fit, many manufacturers do not design a Center Speaker that can truly match the dynamics of the speakers likely to be used as Mains, even if using a Standmount for example (cabinet size is disproportionately small, woofers can be 1-2" smaller, poor port placement, etc.).
A lot of this is seemingly up to the consumer to decide where their values lie, and then shop accordingly (which also entails educating themselves appropriately). This is why I joined the Forum here.
In my own system, I use 3-way speaker all around my 5 main Channels. My Center and Surrounds are the same, and my Center is upright. My towers use different Tweeters, but are the same brand Tweeter. The Mids are completely different, while the Woofers are again the same Brand as each other. My Rears are currently 2-ways using the same Tweeters as my Center and Surrounds.
This is an outlier scenario: I fully understand my own unique reasons and do not expect others to adhere to my idiosyncrasies.
However, I do preach what I practice and encourage those I engage with to think outside the box and look at options that will work better for their tastes than just buying what the manufacturer is slinging.