Good point. I may have been overly critical on this.
But, with the exception of studio work with a technical camera, where it is tethered to a screen right next to the camera, I hold to my case.
Frankly, the cost of a slide back for focusing is tiny compared to the cost of back and most technical cameras, so I don't see this as a cost saving of much merit. The upgrade fee is about $8,000, far more than any sliding adaptor.
Michael