If everything is working for you, i’d say just keep it the way it is. No need to fix it if it isn’t broke. The only reason you may want to upgrade the compatibility to level 10 is that you can up your hard drive space from 2TB to 62TB per drive. We needed to do this for a couple of our servers, but it’s rare that you would need a VM with that kind of space. But then again, you’d need VMWare Workstation or vSphere to edit it from there.
Now if they offered HA for 2 free ESXi servers, just for absolute basic redundancy, i’d be in thrilled. We are on a strong push to move our teams to VM’s instead of having actual boxes at their desk for testing.