It's controversial. Almost anyone can call themselves a pet nutritionist. A lot of the folks in Petsmart are well intentioned but sometimes don't know much, and are coached what to say. I worked for a dog food company for a brief period of time (and stood in Petsmart every weekend) and they give you lines to spout, etc, and don't even really tell you or explain to you anything... it's a lot of "say this, promote this, etc".
I don't personally think corn is beneficial. Those who sell food with corn in it will say it is. Just like those who sell food without it will say it's not. Food is a political thing around dog forums and dog people, LOL.
I don't think corn is all evil or anything that it can be made out to be. But I don't think it's necessary at
all for a dog, or carnivore. Humans can't even digest it. Jackson ate some corn the other day that dropped on the floor and it was in his poop the next morning, LOL. Just like humans.
I like
The Dog Food Project - How does your Dog Food Brand compare? as a reference - she doesn't say corn is "bad" by any means. She just explains a lot of different ingredients, etc.
For my dog, I've found grainfree to work the best for him, thru trial & error and "experiment".