After visiting Sheldon brown's gear calculator, it seems the overall advanatage to lowering the gearing would be negligible. I have a Trek 520 with the 50/42/30 and 11-32 rear. The three lowest combinations in gear inches that I'd get is 33.8, 28.9, and 25.3. I have seen some people say they change to a 26 granny. If I did this, my bottom three combinations in gear inches would be 29.2, 25.1, and 21.9.
So, really I would only be getting one more combination that is of any significance. With a 24 granny, you get two lower combinations of any significance (24.8, 21.2, and 18.6). How many times have you all actually needed your bottom combination to the point that you would have had to get off your bike and walk? Or am I missing something else about the whole gear inch thing?