Hi Bob,
From what I've heard, when a bird eats bread thrown for it, it often swells inside the stomach and gives the bird a feeling as though it's fuller, when in fact it isn't. Subsequently it eats less, with the bread then being less nutritious than what it would usually go for as well. Young birds/chicks/goslings can apparently choke if it swells in the throat. Thirdly, bread thrown into the water can stagnate and in doing so, uses up oxygen. With less oxygen I believe you then get a greater prevalence of algae which covers the surface, and slows down photosynthesis of some aquatic plants in a worse-case scenario! These are the things I've often read - but as you say, conflicting view points.
That said, I think feeding ducks - with bread being the most often used - is an essential way of introducing kids to wildlife, and in some cases hopefully they keep their interest throughout life from it.
Take care,
Jason 