We do use ketones even on a high carb diet, but the absolute values are really low, it's an extremely minor percent of calories burned.
I have kicked this around--dieticians will say our brain "needs" somewhere around 100-120 grams of glucose a day. Other sources give brain metabolism as around 20 percent of total for an adult. For somebody at 2000 calories a day, that gives 400 calories, what you'd get from 100 grams of glucose. Of course this is going to be different for a four foot woman than for a seven foot man.
Anyways--2000 calories low carb...
Give 80 for the popular 20 gram net figure that's going around these days.
Say 100 grams for protein... so we're up to 480 non-fat calories. 1520 calories left for fat, from body or diet. At 9 calories per gram, that's about 169 grams of fat.
Ten percent of the fat is glycerol, you can produce about 17 grams of glucose from that (or that glycerol can be used more directly, decreasing need for glucose or ketones.
The protein--the usual figure is that half to two thirds of amino acids from protein can be made into glucose. So another 50 grams or so of glucose--or even if it never becomes glucose, still a potential brain energy source, either directly, or the amino acids can be used to produce various organic acids that can find their way to the brain and be used for energy.
That's 67 grams of non-ketone glucose substitutes. Add that 20 grams of carbohydrate that's supposed to have you in a ketogenic state as long as you don't go over it, depending who you talk to--we're up to 87 grams of glucose plus glucose substitute. That's already fairly close to the claimed daily energy needs of the brain. So we don't need the brain to secretly be able to burn long chain fatty acids to explain someone eating the low carb diet I outlined without being in very deep ketosis, the stuff's all there, if the metabolism will just cooperate.
Add that a lot of people will be burning more fat than this (and also more protein). Also add in the fact that medium chain triglyceride not only increases ketosis in the liver slightly--but can also cross the blood brain barrier and make its way into brain mitochondria, where it is subject to beta oxidation and can provide energy. There are little pathways here and there--some scientists actually suspect that humans can produce glucose from acetone, Chris MasterJohn has posted about this, it's not enormous, but small inputs become increasingly important the stricter the diet becomes. Also long chain fatty acids can be used to produce medium chain fatty acids, which as I said can be a more direct brain fuel, and a small amount of even chain fatty acids can be used to produce odd chain fatty acids--odd chain fatty acids can be used to make glucose. It's tiny--like, two 22 carbon fatty acids could be used to produce two 3 carbon odd chain fatty acids, enough to produce one glucose molecule. But maybe these little bits add up.