I am 5'11" and my hair touches the ceiling when sitting in the middle seat of the 2nd row.
For reference, my hair also touches the rear glass window when sitting in the 3rd row.
3 adults in the 2nd row of the Model X was a really tight squeeze.
I changed my config from 7 passenger to 6 after sitting in the car.
Others have expressed similar concerns. The counterpoint I have for this reasoning is that it assumes that all potential passengers will be at least a certain size. Even adults are not all the same size. I frequently take 5 adult passengers in my current car which has a rear seat with less shoulder and hip room to the X. The passengers usually sort themselves so that the 3 narrowest are in the back seat and the same thing would happen with the X with even more options considering the 3rd row. Sometimes there aren't 3 that are noticeably narrower than the rest, but we are only going a couple of miles so they deal with it.
The center seat is simply another option for someone to occupy. If you have fewer than 7 people then it doesn't even have to be used (though it does limit leg stretch room for the 3rd row). If you have 7 passengers then it's likely you have at least one shorter than 5'10". If you have 7 people that all happen to be that tall or taller, then they'll still be more comfortable than if you have to leave one behind.
To me, the only reasons to consider the 6 seat configuration are leg room for the rear seat, pass-through cargo, and rear visibility. For me the pass-through isn't likely to be used much and I could deal, and the rear visibility is mitigated by the availability of a full-time rear camera screen. So, it mostly boils down to 3rd row leg room.
Another plus I see for choosing 7 over 6 is that I frequently have 5 passengers and if I had the 6 seat configuration then one person gets isolated in the rear. 7 seats provides much more flexibility for ensuring that everyone gets a seat buddy. If my 5 passengers would prefer to sit 2/2/1 in the 7 seater then they can still do that and the rear passenger may even benefit from sitting a little sideways for more leg room. But, honestly, I really think they are more likely to sit 2+3 so they can talk more. Maybe I just have chatty friends.