The use of reservations is generally only required where memory (or CPU) over-commitment is such that one or more VMs are suffering contention such as ballooning or swapping.
If the VMs are not experiencing swapping or ballooning, check the Active memory verses consumed, and you will likely find the active is much lower than consumed.
The above comments above reservations affecting the HA admission control calculations are also correct, as such avoid reservations unless absolutely required.