Pretty much all the same arguments I use. We could add the ETH, the expensive tissue hypothesis. But basically no argument required, just test for yourself. Eat some plant, check at the other end. If it's not starch, it comes out intact. If it comes out intact, there's little reason to believe we got anything out of that as it passed through our gut.

On a slightly different note, whenever I discuss diet with a veggy proponent, I try to turn the discussion toward supplementing with the things that are obviously lacking in such a diet, rather than merely mock them (which I used to do, but not anymore). So, fat, fat-solubles, B12, that kind of thing.