virtualbox.org

Been researching for days and still can't figure out whenever or not the guest addition is needed if the
guest OS is a headless Internet server, no GUI, no shared folders, no clipboard.

Running a ubuntu 10.04 host 64bit - with freebsd 8.1 guest OS's. Everything seems to be running smooth
but the servers haven't seen much load yet.

Someone please enlighten me

Thanks in advance

There is a dedicated chapter regarding guest additions in the user manual. In short, for your task the only relevance would be probably time synchronization of the guest with the host. If your don't need that (e.g. you are using ntpd) then you don't need the additions.

Correct, I wouldn't need the sync either.

The reason I'm asking is I've seen the phrase "for better performance" related to the guest additions - but the features listed is only for usability.

The increased performance is for when you're using a GUI. Since your VM is headless, meaning it doesn't have a GUI, you won't get that performance increase. As you might know, proper video drivers will reduce the need for CPU rendering and thus gives more performance as CPU cycles aren't wasted for the display.