For all of your systems I would not think that makes sense, though the type of systems you plan to put in place would matter.
To run one one virtual machine on one box has only one benefit, the ability to restore it somewhat quickly if it crashes. But you really are wasting resources the could be better utilized toward server services.
You say the that the backup is easier, but is it? You still need to backup your host machine. I mean you don't want to have to reload it from scratch, then load all the patches before you can get your VM up and running? What are you saving there?
Invest in a true backup software application like Backup Exec with it's Disaster Recovery add-on, this will allow you to reload servers straight from tape if they need to be rebuilt.
If you are running 2 VM's on a machine, maybe it's worth it, but probably not. You are still better running the services right on the host OS.
Perhaps I (we) could give you a more direct answer is you could give some details on the environment you are thinking about visualizing.
One point I thought of, is that recovery could be moved to a machine already setup up a Hyper-V host, very quickly