It's a good idea to start studying GNU/Linux in VMs. And you have chosen a
good hypervisor for Windows. VirtualBox is almost free/libre as well as
beginner-friendly. In my practice, I'd also like to suggest beginners that
they start with VirtualBox.
If the Guest Addition ISO file is included in the main binary file (released
under GPL v2) but not in the Extension Pack (non-free), then there's no major
freedom issue.
However, I never used the Guest Additions in practice. I just checked the
Section 4.1 of the VirtualBox manual describing said Add-ons. On GNU/Linux
guest operating systems, I don't really need the features provided by the
Add-ons. In other words, I can easily find better alternatives. Let's talk
about the features provided by the Addition pack:
1. Mouse pointer integration. Any modern GNU/Linux distribution should
support this out-of-box.
2. Shared folders. Just install SSH server on the GNU/Linux guest operating
system, and you should be able to share files securely. (You need to set the
NIC to non-NAT mode, or else configure port forwarding. I set two NICs for my
VM. One is set to NAT mode to access Internet, and the other is set to Host
Only to share files with the physical host and/or other VMs.) Use free/libre
software like FileZilla under your Windows host to access your SFTP server in
the VM.
3. Better video support. Even with this addition, you can't expect to use
your VM as a gaming console.
4. Seamless Window. X11 forwarding is way better. Just log on your VM using
command "ssh -X" (or maybe "ssh -Y" if you use Cygwin on your Windows host)
and you can project any window to your Windows host. (Be sure that you are
running Xorg rather than Wayland on your GNU/Linux VM.)
5. Generic host/guest communication channels. Same as 2, SSH is way better.
Just configure your virtual NIC correctly. Use free/libre software like PuTTY
or Cygwin to log on your VM.
6. Time synchronization. Totally useless if you have Internet access.
7. Shared clipboard. This is a bug, not a feature.
8. Automated logins. This is a bug, not a feature.
