I don't think its an issue of [white] rice not being digestible (it is!), its a matter of it having very little nutritional content to the dog when digested.
Rice is a high-starch carb. And while dogs can benefit from some carbohydrates in their diet (they get the majority of their usable energy from fat), including a large amount of them is, more or less, wasteful. Its like eating a lot of empty calories.
Basically all commercial pet foods do this. Just like in the Duck can here, I'm sure a good deal of the calories are coming from the sw. potato, but I'm not overly worried about it. I doubt the amount of rice/sp. potato/etc in these cans makes up the majority of the product as it does with most kibbles, which tend to be much higher in starches and carbs on the whole.
GSDs- Mulder, Ridley, Vega
GSD on the half shell- Rigel