App guys always want all the memory! Their perspective is protecting the performance of their application, at the expense of the performance and availability of the environment!
You have to tell them NO, because this setting may preclude HA from functioning correctly. Moreover the use case for reserving all the memory on a VM is protecting one/several VMs among many. When reserving all the memory for the majority of VMs, it becomes self-defeating. Protect your infrastructure and your mission-critical apps will run better as a result!