Many comatose individuals will never wake up, and have already suffered significant brain damage. Keeping these people in a coma is cruel, and only prolongs the suffering of them and their families. But if they are allowed to make this decision, it will not only save suffering, but reduce medical expenses.
When there is no DNR or living will to follow, doctors should follow the wishes of the patient's next of kin in regards to life support and life sustaining procedures in regards to a patient who is comatose. A spouse, parent, child, would be more aware of what the patient would want and would not take such a decision lightly.
I believe that as the law stands in most states, doctors are allowed to halt life-saving measures if the individual's health proxy makes that determination. Doctors now have the ability to see the level of brain functioning and to closely evaluate the level of function a person will have if they wake from a comatose state after a trauma. If the doctor estimates very low function, the health proxy should be able to make this call.
If a person is comatose and there are no signs of life, and when a doctor states that the medical evidence shows this person will likely be this way forever, then yes, they should be taken off of life support. Just being alive doesn't give you a life. Lying in a bed, connected to machines, never talking, walking, eating, seeing, hearing, or moving is no way of life, and should not be happening. Mercilessly let the person go to a better place, free from suffering and lack of livelihood.