The Scary Idea is certainly something to keep in mind
He doesn't seem to be making the argument you describe anywhere near the cited quote.
Say your options are: Stop and develop Friendly theory, or continue developing AI. In the second option the utility of A, continuing AI development, is one utilon, and B, the end of the existence of at least humanity and possibly the whole universe, is negative one million utilons. The Scary Idea in this context is that the probability of B is 1%, so that the utility of the second option is negative 9999 utilons. If Ben 'keeps it in mind', such that the probability that the Scary Idea is right is 1% (reasonable - only one of his rejections has to be right to knock out one premise, and we only need to knock out one premise to bring the Scary Idea down), then Ben's expected utility is now negative 99 utilons.
I conclude that he isn't keeping the Scary Idea in mind. His whole post is about not accepting the Scary Idea; for that phrase ("pointing out that something scary is possible, is a very different thing from having an argument that it's likely") to support his position and not work against him, he would have to be rejecting the premises purely on their low probability, without considering the expected value.
Hence, the argument that since it's unlikely, we should not bet on it.
Edit for clarity: A and B are the exclusive, exhaustive outcomes of continuing AI development. Stopping to develop Friendly theory has zero utilons.
Ah, Pascal's wager. And here I thought that I wouldn't be seeing it anymore, after I started hanging out with atheists.
[...] 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.