It's funny, every vet I've ever been to has told me this, and yet, I've gotten the best results from a diet of raw meat, no grains, limited veggies. Rice, oatmeal, etc. cause anything from liquid poop to blood filled butt explosions
My point being, having dealt with these issues for a few years now, my own experience tells me that not only do high carb presciption diets fail to control the poop issues in a satisfactory long term manner, but what good they do do is so far outweighed by the cost to the whole dog that it's not worth it.
It's also been my experience that chronic GI problems are a weird thing, what will work for one dog might be a disaster for another. By extension, the idea that a specific type of diet will work for all dogs seems crazy to me.
In general, dogs with GI issues DO tend to respond to easier to digest, low fat, less dense foods. Prescription diets have the research to back them up. That doesn't mean they work 100% of the time either... and just like you've seen, sometimes it does take extreme measures to find a diet that really works for a dog.
With that said, this diet IS working for this dog, and that in itself should be significant player in the food choices that the owner makes in the future. Thats why jlphilli and I made the suggestions we did...