Very good point. The 5.1" usable space of the iPhone X means a smaller screen. Maybe during certain situations (games, full screen video playback, etc), the screen utilizes the full 5.8" screen?
Either way you cut it, the customer loses out when Apple fragments features and options like this. Depending on what they ultimately decide, if you get the 7S, you could lose out on certain features of the Plus and certain features of the iPhone X. If you get the 7S Plus, you could lose on certain features of the iPhone X, and potentially your aforementioned larger screen usability.
Once again, Apple forcing their customers to pay top dollar to get everything you might want; some of which the competition have been offering for some time now.
The whole idea of giving the iPhone 7 a modest update was to celebrate the iPhone's 10th anniversary a year later with crazy new features. 100% agree with you that this celebration should be across their entire 2017 iPhone line up, instead of only in a third, higher, and even more expensive tier.
They're rumors for now so we will see. As I said earlier, it'll still sell like hotcakes, no doubt, but for the customer? Kind of sucks, in my opinion. I've long warned that Apple's pace -- at the level of the technology -- was too slow. And customers are the ones who will suffer for this.