vSphere - Why upgrade VM Hardware Version?

In general, virtual hardware versions introduce new functionality, extend limits and may have performance implications. See the VMware hardware version matrix.

Don't worry about this for the revision of vSphere you're on now. You can run all day on old versions, based on the setup you have. VM hardware version 8 sounds like the best choice for your specific situation.

The only real consideration with regard to virtual hardware versions is the move from version 8 or vmx-09 to the vmx-10 introduced in vSphere 5.5. There are manageability implications of this move. But on the positive, that process is streamlined through the vSphere Web Client, which allows you schedule the VM version upgrade during guest reboot.