We don't need carbs to live and breath, but they are the preferred energy source by the body, especially if you're active and exercising regularly.
An athlete going on a ultra low carb diet is only going to hurt their performance.
I think you are missing the point, saying carbs are the preferred energy source of the body is like saying your car prefers the first half of the tank as it uses it first. Your body is essentially lazy trying to conserve energy as much as possible, so it uses the readily available fuel first, in the absence of carbs the body will use fat as it's preferred fuel.
Re read what I posted earlier and understand it without jumping to extremes, you are the one oversimplifying things by saying what you are saying above. If you are active and exercise regularly, this then obviously comes with the ability to consume extra carbs, should you choose to go that way.
Your body (especially of you are training with weights or are some sort of athlete) must have a certain amount of fat and protein and it must have a certain amount of overall calories, these things are already known, so now you can now work out how many carbs you 'may' consume to make up the rest of your calories.
If your job is sitting at a computer all day or driving a truck, your ability to consume carbs and not get fat is much less than someone that works as a builders labourer digging trenches and carting bricks all day.
A guy who trains for 30-40 minutes 4 times a week with weights (me for example and I also have sedentary job most of the time) has a much lower ability to burn up extra carbs that say a guy who runs 10km a day, does 2hr marathon gym sessions every day of the week or trains for triathlons or what ever.
No one is saying carbs are bad per se, but too many carbs for your level of activity are definitely bad.