It must be great to know no-one within your environment will ever do anything wrong. Or download anything that will do anything wrong. Or visit any websites than can hijack your browser into doing anything wrong.
Being 'private' only means you don't get hit by drive-by scans from the Internet. There are (depending on configuration) plenty of opportunities for internal attacks, for example the workstation being used to access the boxes. Not to mention removable media (usb, cd-rom) or files copied onto those otherwise isolated hosts could be infected.
Patching servers is just good practice. As is designing a system that can handle rebooting individual servers without user-facing downtime.
Only ones that affect the network surface footprint, so none here as it's on a private VLAN.