Jeff has actually spoken about this a few times. The ghost sites do have a cost beyond just resources. The stack exchange format only really works when you get a certain critical mass of people to ask questions. They have found if you get people together and people ask questions then people will come to answer them. The trick is getting enough people together to start asking questions.

When you have an empty site and someone sees the last question was weeks or months ago they just browse away. Helping getting the community over that initial hurdle is important.