A used retina iMac probably is worth more if used as a computer as if a lower resolution display, ie, it would be cheaper to sell a used 5K iMac and buy an external monitor than using the 5K iMac as a lower resolution monitor.Not less expensive when you already have an iMac.
Also, they could have allowed the 5k Retina iMac to run as external monitor at non-Retina resolution? (Basically x0.5)