A camping grill is not the indispensable cooking utensil in the wild, at least not anymore.
Back in the day cooking over a fire was your only option for eating hot food, and a grill was the lightest thing you could carry for cooking. Cooking over a campfire is still fun, it's nostalgic for a lot of people, but it's not necessary in the backcountry these days, in fact it's often discouraged in the backcountry because it isn't in perfect harmony with leave no trace ethics. Small, light, and efficient camp stoves leave no impact on the environment, whereas campfires often encourage breaking branches, cutting trees, and leave a scar the ground. Not to mention the fact that grills leave your backpack really black and dirty if you don't clean them well before strapping them to your bag...
Where cooking over campfires is still acceptable is in developed campgrounds that have designated fire rings and supply wood for burning. Feel free to enjoy the romance of a fire in these settings, but when venturing out into the wild, or exploring back country trails, please plan your meals to be cooked with a backpacking stove.
To answer your question: "So when does a grill become indispensable for cooking over a campfire?" A: Whenever you decide you want to eat something grilled verses eating a tinfoil dinner, or: when you run out of tinfoil.
I think the ultimate indispensable camping cookware is still a cast iron pan, as it has been for hundreds of years, but not many people carry those in the backcountry anymore unless they've got a pack horse to carry it for them.