Indeed this is an interesting debate and one that has taken us so far down several paths. It is interesting that espresso in its purest form is something that has changed very little over time. The Italians are so passionate about protecting the identity of espresso that a few years ago there was a sustained effort to stop espresso being called espresso unless> it was 25-30ml, had crema, was extracted in 25-30 seconds and served in a demitasse cup (maximum 70ml capacity). Obviously on the other side of the equation the new world has worked on breaking the mould and experimenting not just with espresso but also the drinks that they go into. Sometimes this traddition meets the new world does not work (like anything Machiatto should be espresso marked with steamed milk- NOT a milky monstrosity ala SBUX). Regarding the original arguement, I am pretty sure that James who started this debate may have been just stirring the pot...which, for sure, he has done!
Regarding 100% robusta based espressos, I have tried these in Italy. Often the robusta is a blend of several robustas- washed and naturally processed. I am not a fan off higher than 10% robusta in an espresso blend, but I must say one Italian Gold medal blend I tried was excellent at 50:50- using washed robusta from India. Some of the robusta from the sub continent and also from Madagascar is good enough to be blended fairly successfully at a good percentage against Indian, Kenyan or Colombian Arabicas in particular% robusta is, I confess, too much for me. In Naples- where lever machines are predominatley used, the extraction works fairly well with this type of blend and the espresso is palatable. Using pump driven machines, almost undrinkable. I am pretty sure it may explain the number of levers in cafes throughout Southern Italy.
Perhaps my only slight disagreement with Ellen would be I dont subscribe to the point of view that blending as the customer wants is the way to go. I think the roaster has a certain responsibility, being the one with the experience and knowledge of coffee, to be making the call on blends and explaining why. Taste is indeed subjective, but a roasters understanding of his/her beans and how they work together is far more objective, than subjective.