In the Least Convenient Possible World of this hypothetical, each and every dust speck causes a small constant amount of harm, with no knock-on effects(no increasing one's appreciation of the moments when one does not have dust in ones eye, no preventing a 'boring painless existence,' nothing of the sort). Now it may be argued whether this would occur with actual dust, but that is not really the question at hand. Dust was just chosen as being a 'seemingly trivial bad thing.' and if you prefer some other trivial bad thing, just replace that in the problem and the question remains the same.