Allowing ferrets free access to the house

Question
My girlfriend and I live together and are having a significant disagreement about how our three ferrets should live.  We purchased them as kits this past winter (I'm writing this mid-October) and although the original plan was to have them live in a three-story Ferret Nation cage with frequent opportunities for supervised play outside it, once she realized how intelligent they were she decided they should NEVER be confined to the cage. This gives them free access to an un-ferret-proofed 2700 square foot raised ranch house.

My view is that in addition to being dangerous this is very confusing for them. For instance, in such a large space with so many different rooms, closets, nooks and crannies there has been no way to housetrain them, so they relieve themselves in every corner of every part of the house. They also chewed their way into the living room sofa and use that to hide in (and conceal whatever treasures they want to hang on to). If a bureau drawer is left open they will nap on top of the clothes and urinate and defecate in the bottom of the bureau behind the drawer. They despise the feel of litter under their feet and will defecate NEXT TO the litter boxes in preference to IN them.  We have had a little bit of success with disposable pads similar to what one might use to train a puppy, and they use the pads increasingly but not reliably.

Fortunately they haven't gotten hurt.

On routine vet visits the vet, who is very ferret-experienced, has commented that these are the happiest, best-adjusted, most even-tempered ferrets she has ever seen in her life (she hasn't been told of their living conditions). Most ferrets have to be scruffed for certain things, like trimming their nails. Ours will sit quietly and allow it. My girlfriend attributes this to the permissive atmosphere.

I suggested a compromise in which we put a ferret gate at the top of the stairs to confine them to the upper floor (the upper floor is hardwood; the finished basement, with its carpeted floors, is their favorite unauthorized poop place and obviously the hardest to clean because of the carpeting). I figured this might reduce the area to one in which maybe they could be litterbox trained. She won't hear of confining them to the upstairs, saying that they are sufficiently intelligent to impute messages from our actions and that confining them to only part of the house will negatively impact them psychologically.

Don't get me wrong: having owned both dogs and cats in the past, I think these are the most wonderful pets imaginable and I love them with all my heart. But I don't think this is the right way to raise them.

What say you?

Answer
There are a number of reasons I would not allow ferrets free roam of an entire house.  First is safety.  I have heard too many stories about ferrets getting hurt and/or killed in seemingly mundane circumstances around a house.  Of the ferret can escape through an open door or even a dryer vent.  Secondly, I believe it is how you use space and how you interact with an animal that gives it psychological impact, not the amount of space.  Think about a small bedroom that has books, movies, friends, etc or a large bedroom with only a bed.  Size is not everything.  As long as there is ferret enrichment and interaction you will have healthy ferrets.  We use tubes and rubbermaid boxes to keep their environment similar to that which they would find in "the wild".  Lastly, you are right about litterbox habits.  In my experience, the more space a ferret has, the less "good" they are about using the litterbox.
As a final thought, if there were ever a fire, how would you collect them all quickly?  I believe ferrets need a specific ferret space that is designed to meet their needs and one that is designed to keep them safe at all times.