A good question, thanks for posting it.
This is one subject that I keep changing my mind on. If people would only do it in moderation, I don't think there would be much if any problem, what I detest seeing is people arriving with carrier bags full with loaves of bread, all of which will be left around a pond or park, often feeding the birds right next to the sign telling them the dangers of overfeeding bread.
There again, to see a mother with a couple of very young children and the delight it gives them feeding the birds with just a few slices, how can you argue with that?