You do not need BMI to know whether someone is overweight. Just look at them. Before BMI doctors twice warned me to lose a bit of weight (about 7lbs) and that was based on just seeing me in a state of partial undress in the consulting room. Photographs of me then show that I was indeed getting porky.
I also think the approach to helping people with weight problems is unhelpful. I watched a tv programme where doctors were 'helping' potential bariatric patients lose some weight and change their eating habits. One day they were at home eating 4,000 calories a day, a diet rich in sugar, fat and refined carbohydrates. The next day they were in a hospital receiving 1500 calories a day, a diet short of sugar, fat and refined carbohydrates and rich in vegetables and unrefined carbohydrates.
The failure rate was nearly 100%, which didn't surprise me. Those with weight problems and poor diets need to be helped to shift from a bad to a good eating pattern gradually, replacing sugar drinks with diet drinks, full fat milk, via semi-skinned to skimmed. They need to be taught to cook, how to make low-cal versions of their favourite take-aways, helped to make small changes that gradually move them towards habits that will bring their weight down and last a life time.