I look at it from the pedestrians' perspective. In a 20mph zone, they are entitled to expect that the limit is there to prioritise their safety and that it should be respected by all vehicle operators. The elderly, and others who are less mobile, should be able to cross streets in these zones without the worry and apprehension that comes with doing the same in a 30mph, or higher, area. Crossing those roads is stressful, worrying and tiring, as they hurry as best they can to get to the other side.
And I don't expect the average pedestrian to know that speed limits can't be enforced against cyclists.
As infrequent as the opportunity to exceed it may be, I will always comply with the posted speed limit while cycling. While it might not be my legal obligation, I feel it is my moral/societal obligation.