Apple does not expect you to use a case, though they will certainly sell you one. Virtually no Apple promotional media features people using iPhones in cases (they are always au naturale), and Jony Ive certainly does not use one.
The cases Apple sells you feel like they're part of the phone after you put them on. My phone feels naked without it. And I was a hardcore case hater for the longest time. Now I've seen the light, Apple engineered their cases with every bit as much care as they did the phone. I can see why they wouldn't put one on in the marketing materials, but trust me they really do go together.
Check out the left edge of your iPhone X. Look at the ugly cutout for the mute toggle, right above the plastic buttons they added to pad out the phone's volume buttons so that they're actually usable in a case. If "in case" was the design spec for the iPhone, I think Apple could do better here.
I have a 7 Plus, but mine has a cutout as well for the mute toggle that makes it more difficult to use. I consider this a feature, when I went without a case, one of my main gripes was that I could never control the state of the mute toggle. Now it never switches on or off accidentally while the phone is in my toggle.
Just because they decided "if you insist on using a case, we built you the best case" (and I agree, and I use the case; but I also used cases from InCase before Apple started making cases), that doesn't mean they intend you to use a case.