When using resource reservations (even if on just a single virtual machine out of many), VMware will apply the lowest common resource reservation to all VMs. So if VM A has x resource reservations and VM B through VM E don't have resource reservations, VMware will apply x resource resolutions to all virtual machines.
By using resource reservations, you may be artificially limiting yourself from taking advantage of the full capability of your environment. Techniques like oversubscription can no longer be fully utilized when using resource reservations.
If your cluster is healthy with plenty of resources to share between virtual machines, we recommend disabling resource reservations. If you are constantly seeing issues with insufficient resources to your cluster as a whole, you should increase the number of hosts in your cluster or decrease your workload.