Speaking as Nanodevil's Advocate again, one objection I could bring up goes as follows:

While it is true that applying incomplete knowledge to practical tasks (such as ending the world or whatnot) is difficult, in this specific case our knowledge is complete enough. We humans currently have enough scientific data to develop self-replicating nanotechnology within the next 20 years (which is what we will most likely end up doing). An AI would be able to do this much faster, since it is smarter than us; is not hampered by our cognitive and social biases; and can integrate information from multiple sources much better than we can.