In Yudkowsky's argument, it's the option between one person _absolutely guaranteed_ to have 50 years of torture, vs 3^^^3 people _absolutely guaranteed_ to be dust-moted. I think the guarantee changes things significantly.
Just think of it as if you were going to other people and deciding to take this bet for them. 1/3^^^3 chance of torture in exchange for removing a speck of dust from their eye. I think you would be ok with that because it's an obvious choice for ourselves.
And if you continue to do this for enough people, eventually one of them will get tortured. After removing dust specks from approximately 3^^^3 people's eye. But you are right, there is a chance no one will get tortured. So do it for 3^^^^3 people then and the probability is 0.99999...
If you were going to live for 3^^^3 lifetimes, would you like to be tortured for 50 years in exchange for one fewer dust specks in your eye during each of those lifetimes.
I think most people would answer no. So the debate has nothing to do with morality in general. Instead the debate is about Scope Insensitivity (http://lesswrong.com/lw/hw/scope_insensitivity/), that our emotional reaction to things isn't diminished by even infinitesimally small quantities (see also Useless Medical Disclaimers http://lesswrong.com/lw/h4/useless_medical_disclaimers/).
I think I can make that position seem more reasonable if framed another way: Would you risk a 1/(3^^^3) chance at 50 years of torture, in exchange for getting rid of one dust speck out of your eye? I think most people would say yes. You couldn't even get out of bed in the morning if you weren't willing to take even incredibly small risks.
There is a much higher probability than 1/(3^^^3) that you could get in a car accident with injuries that cause 50 years of incredible pain, yet you will still probably risk driving for even trivial things.
And if you lived for 3^^^3 lifetimes and took this risk each time, you would likely suffer 50 years of torture during at least one lifetime.