Medical apps are true high requirements yes - specially when interacting with other devices. But also games, depending on what you are doing, you definitely want to be testing on multiple devices, specifically if you are pushing the hardware in some way performance wise. When building game engines you will also be getting different reports with different qualities from other developers, and having real devices really speeds up reproducing issues.

A version that would run without batteries would be super sweet! I understand such dev-kit for phones/tablets may not benefit from economy of scale and may be expensive but it would be nice to have the option. They could be built to be stackable, so you put them in a furniture or maybe even in a rack of devices. But once you are developing for phones and tablets and you have one device on each system version, different sizes and all, it gets really boring getting each of the drawer and charging and testing and all. Once in a while I see some offer of a platform that has these things remotely accessible through as a service, but it's not the same as having the devices available for real touch.