I never noticed that behaviour because I only use mission control in full-screen mode. If you swipe up with three (or four) fingers from a full-screen window the previews are visible immediately. I have no idea why we need a different preview for desktop vs full screen however.
The part of this UX that annoys me is the spaces get re-ordered for no apparent reason. I usually have a few IDE windows open and it's tiring to have to double-check the window hasn't moved.
The non-fullscreen (desktop) case uses an animation for the same purpose, locating the current app window in a sea of others.
So what would the preview be in the swipe-from-desktop case? A preview of the window-sea, or the desktop as is? What should the animation be? I suspect those questions are why they chose to just name the desktop.
I think it would be more consistent if the tab based preview only existed for the desktop window-sea and transitioned to the actual space previews when swiping between spaces.
Previews are also visible immediately if you set Mission Control as a hot-corner action. In never see the title-only spaces — i forgot it even did that until this discussion.
I also wish I could name the Spaces. "Desktop N" is pretty useless.
However, swiping beetween the previews, it sometimes jumps to random places in the order - which is not nice.
Possibly a bug, but I might as well just write this as a letter to Santa because it's got more chance of being read than a feeback.
The setting is "Automatically rearrange Spaces based on most recent use" which explains why the behaviour felt so intermittent.
Wouldn't it be great to have them named "Design", "Dev", "Productivity", "Games". Or whatever makes sense given your needs, instead of simply desktop #.