I am not a nanny, but here is my perspective as a child of divorced parents. My parents had joint custody and I lived 50/50 mom/dad. We always had a live-in nanny. There was no way that my dad could have raised me, working full-time in the job that he had without full-time live-in help. Our house wasn't big (well, until he got remarried and they got a big house and had more kids), but our nanny's room was on the lower level with a completely separate exit/entrance and own living space. Our nanny actually moved with me when it was time for me to live with my mom (a mere two miles down the road).
There are so many divorced parents out there, and I think it's great when dads take an active role as my dad did. It would be a shame for them not to be able to find a nanny.
So, IMO, as long as there is separate living space, I don't see the issue -- sure, I see a concern, but how is that different from working for a married couple where the wife travels or a bad marriage, etc?