First of all, it makes the client feel like the system is inherently unsafe and inferior to other solutions and this is not true.
But it has always been recommended practice to have a fallback hardwired POTS phone, regardless of the PBX in use and whether it has a battery backup or not. You just want the fallback, in case everything else goes down. Maybe it's less important now with everybody carrying a cell phone. But moving from proprietary PBX to * is not what makes this a requirement, or at least a recommended option.
