I think a lot of people still cling to the belief that a human can always do certain tasks safer than a machine -- and rightly so IMO. To date I have yet to see an IPS that did the right thing when it found a false positive.

It does take a lot of skill to set up any such systems -- just like IDSes. I think it will become more evident as time goes on that networks are a lot like plant or animal ecosystems (I remember reading an article about something along these lines recently), and with that comes the concept that a fluid environment can't easily be predicted, nor can it be easily replicated. Any two networks with the same given size, computers, and hardware throughout will still vary greatly in terms of usage, environment, etc... I would certainly think that hiring someone to initially configure such systems would increase the cost of deployment to beyond the point where it's worthwhile to have an automated network security analyst, especially when you can just hire one.