There is nothing fundamentally bad about using carbs for fuel. The main problem is a pragmatic one. There are only three fundamental sources of carbs: 1) fructose containing carbs like sugars and sugary plants (fruits). 2) Low GI glucose polymers (starches) that all contain proven inflammatory compounds that raise cortisol and increase the risk of autoimmune disease. These would be whole grains and legumes, and most particularly, high gluten wheat products which ALL contain proven inflammatory compounds (in fact the inflammory fibers, lectins, and proteins are the very reason why they are "low GI". and 3) high GI glucose polymers that can cause blood sugar excursions both in the high (140+) and low range, both of which are linked to greater health hazzard ratios.
If you don't get more fructose than your liver can manage per day, you avoid the proven inflammatory starches and you manage the high GI starches to avoid blood sugar swings (either by consuming them around periods of high activity, or in combination with fat, or in small doses throughout the day) then carbs are fine. Unfortunately there is currently no such thing as a low GI, non-inflammatory, low fructose yielding carb source.