For us there have been two basic factors why we have no living-in maid:
1. The privacy issue (always)
2. Lack of trust
We have got a part time maid for a few years already and actually the lack of trust comes from our own experience with them and observations how many of the living-in maids behave "outside" - less from the horror stories. It ultimately comes to what person you get and a silent assumption that majority of them are good (work attitude + personality) is not necessarily correct. People like to write a lot about respect and this kind of things but this really does not help too much as we always have been respectful, often coming with some extra efforts to make their work more easy and this was always abused. No experience with living-in maids so far but I see no clear reasons why it should be THAT different comparing to the part time maids. Yes, probably the l-in-m should be less spoiled but this would be probably it.
We are taking one l-in-m soon because we are both working and the little one is already with us. If not, we would never decide to do it. Just all the convenience mentioned by MHB and k1w1 is not enough to level the two points I mentioned.