So I looked at the source/targets/virtualbox-host.target file from which the bundle is created, and, IMHO, there are some issues with it.
That's not surprising. Running as a virtualbox host is not something that is well tested. As you can see from the roadmap WWW page, there are a few things that are hard to test because I simply don't have them in use anywhere, like GELI and AppleTalk. Another of those is a machine hosting virtualbox VMs.
But on a more conceptual level, I don't really understand why these 'Wants=' lines are even there.
They might well be entirely redundant, as you say. I'll add looking at them afresh to the to-do list. It may well be simply the case that they can be deleted outright from the target, and enabled by presets. That's certainly the design on the running-as-guest side. (There seems to be a slight bug in 80-virtualbox-guest.preset, considering this. Hmmm.)