As food, plants are utterly unnecessary. I posted some links in s/Carnivore where you can find a lot of reports, many of them hard science, as to how this is factual.

I'm not saying no plants can ever help us: I use hawthorn for cardiovascular health and devil's claw (harpagophytum procumbens) for my tendinitis problems. As medicine, plants are great IMHO. But not as food. And the reason is simple: pretty much all plants contain antinutrients: things that prevent absorption of nutrients. As such, you might have calcium in your broccoli, but something that blocks its absorption too. Meat is utterly devoid of antinutrients, meaning you can absorb all of its goodness.