It's already been said that dogs are opportunistic carnivores and scavengers. They will in fact eat just about anything.
Look at a dog's teeth and jaw structure. There are no flat molars for grinding plant matter, the jaw opens only up and down, it does not move side to side like other animals that thrive on plant matter. They have teeth build for ripping and tearing meat. Jaws built to chomp bones. It makes sense, from this standpoint, to say that dogs are built to thrive best on a carnivore diet.
Then onto the digestion process. It's important to remember that digestion begins in the stomach for dogs, and chewing is merely a means to get the food to fit down the throat. Dogs are not capable of breaking down fresh, raw plant matter on their own. They may be able to extract little nutrients from cooked pureed veggies and fruits, but generally all adding that to the diet does is add bulk, not purpose. To me, if cooking and mushing food in order for it to MAYBE be utilized is required, than that's enough to tell me that it's not naturally intended. When was the last time you saw wolves with ovens, blenders, and microwaves? To me, the point of going with a home prepared diet for my dogs is getting away from things that are unnecessary, and stick to things easy and natural to digest: meat, bones, and organs. If I'm going to start blending, cooking, mushing, etc... well, there's kibble and pre made diets for that business. Annie loved green beans. LOVES them. All summer, I picked them out of my grandma's garden, and she adored them. But, rest assured, every time she ate one, it came out looking the same as it went in, which tells me she was able to utilize none of it. NONE.
Then there's the argument that if a dog eats it, there must be something in it that they "need" or are seeking out.
I think that often times, we like to think that are dogs work entirely off of instinct, but it's simply not true. Just as you or I eat things for the sole reason that it tastes dang good, so do dogs, cats, and any other animal. Annie is a huge fan of pudding!
I have a cat that will not TOUCH anything but kibble, and fruit snacks. Yes, fruit snacks. She will not eat canned cat food, she won't eat raw meat, but she will eat fruit snacks. (obviously not something we give her, rather something she tries to steal)
I don't believe that most fruits or veggies would do much harm to a dog, by any means (starchy foods is another story) but everything I know about dogs tells me that they can survive on an omnivore diet, but thrive and obtain optimum health on a carnivore diet. So, that's what I feed my girls.