The comparison with spice-vdagent is only ~0,3 MB. That useless ~0,3 MB for VBox users is a good compromise for also vastly improving platform support (KVM / libvirt). Very small size, very high benefit.
sudo apt-get install qemu-kvm libvirt-bin virt-manager
Would cause extra 241 MB. Really big size, dubious benefit.
There is also a package anon-workstation-extra-applications (https://github.com/Whonix/anon-meta-packages/blob/master/debian/control#L154). (https://www.whonix.org/wiki/Software)
sudo apt-get install shutter gtk-recordmydesktop libreoffice kdenlive kolourpaint4
Would only cause extra 54 MB. And those would be more justified.
The problem I see installing qemu-kvm, libvirt-bin, virt-manager by default is seeing complaints why image size increased, why update size increased, difficulty for justification why they are installed and what to answer to a feature request "shrink size, remove qemu-kvm, libvirt-bin, virt-manager".
Making a good choice for default applications is difficult and controversial, but I am not convinced of the benefit of those. When there is a big demand, it's easier to justify.