The talk about uncertainty is indeed a red herring. There are two things going on here:
A linear aggregative (or fast-growing enough in the relevant range) social welfare function makes even small probabilities of existential risk more important than large costs or benefits today. This is the Bostrom astronomical waste point. Weitzmann just uses a peculiar model (with agents with bizarre preferences that assign infinite disutility to death, and a strangely constricted probability distribution over outcomes) to indirectly introduce this. You can reject it with a bounded social welfare function like Manzi or Nordhaus, representing your limited willingness to sacrifice for future generations.
The fact that there are many existential risks competing for our attention, and many routes to affecting existential risk, so that spending effort on any particular risk now means not spending that effort on other existential risks, or keeping it around while new knowledge accumulates, etc. Does the x-risk reduction from climate change mitigation beat the reduction from asteroid defense or lobbying for arms control treaties at the current margin? Weitzmann addresses this by saying that the risk from surprise catastrophic climate change is much higher than other existential risks collectively, which I don't find plausible.
[...] SIAI's Scary Idea goes way beyond the mere statement that there are risks as well as benefits associated with advanced AGI, and that AGI is a potential existential risk.
[...] Although an intense interest in rationalism is one of the hallmarks of the SIAI community, still I have not yet seen a clear logical argument for the Scary Idea laid out anywhere. (If I'm wrong, please send me the link, and I'll revise this post accordingly. Be aware that I've already at least skimmed everything Eliezer Yudkowsky has written on related topics.)
So if one wants a clear argument for the Scary Idea, one basically has to construct it oneself.
[...] If you put the above points all together, you come up with a heuristic argument for the Scary Idea. Roughly, the argument goes something like: If someone builds an advanced AGI without a provably Friendly architecture, probably it will have a hard takeoff, and then probably this will lead to a superhuman AGI system with an architecture drawn from the vast majority of mind-architectures that are not sufficiently harmonious with the complex, fragile human value system to make humans happy and keep humans around.
The line of argument makes sense, if you accept the premises.
But, I don't.
Ben Goertzel: The Singularity Institute's Scary Idea (and Why I Don't Buy It), October 29 2010. Thanks to XiXiDu for the pointer.