I have not heard that this is known to be a reason dogs do that - it's more common for them to eat their own feces, which can't have any gut-culture benefit - but it does work in humans, where it's called a "fecal transplant".

How can you prefer one thing over the other if you haven't tried both? It's so funny how we are set up to think we are in control but overriding the default here would take some serious effort. Who is in charge really? The gut microbes order specific kinds of food then we obey? Trying to refuse to eat or change the order is pretty hard.