I'm not sure how much (if any) they would consume on their own in the wild, but the animals they eat do consume them. It would be a very small amount, and since the grains/plants would be partially digested in the stomach of their prey, it would be much easier for the cat to digest the rest.
The biggest problem with grains in the commercial food is that companies use them as the main protein source, rather than animal based protein. We all know that some grains have excellent protein quality, but not for an obligate carnivore. Simply stated, they just aren't designed to metabolize large amounts of plant based protein. As a result, many cats fed on a primary diet of grains become overweight. They don't use grains for energy sources, like humans do, so they are just stored as fat. They use protein and fat as energy sources, which makes sense because the animals they are eat comprised of meat and fat.
The fruits and veggies added to the diet of some commercial foods aren't added to be a protein source. They are added for overall bulk, and because they do have some minerals and nutrients that can be helpful.