I once met a chap who told me he used his dishwasher without any powder or soap at all, and it still worked.

Now, maybe he was making it up. But i suspect that there is a common class of dish dirt where prolonged steaming and jetting with hot water is adequate in practice - i'd expect anything water-soluble to dissolve in hot water! So, residues from tea or coffee, water-based pasta sauce, fruit juice, jam, etc. It won't work for everything, of course, but it might work for a surprising range of things.