The idea behind BRAT is that none of the foods included are not particularly harsh on the gastrointestinal system, and are low in fibre. This is mostly true, although intolerances and allergies to Apples are reasonably common. However, there's no particular evidence that this aids recovery. Patients should eat a diet that is gentle on the system and that they are happy with. The most important treatment from diarrhea is rehydrative treatment. Treatment in the main does not focus on relieving symptoms, instead focussing on preventing harm from dehydration, which is the biggest hazard.
The actuality is that BRAT provides a far from balanced diet, and is more of a tradition than a scientifically grounded diet. Although the Carbohydrate content is adequate, as are some other nutrients like Potassium, it lacks in many areas, including fibre - and the basis of it was that a low fibre diet would help avoid further upsets. Whilst a weirdly unbalanced diet won't hurt for a few days whilst a patient recovers, it's not going to help particularly, although it is better than eating processed food.
Most diarrhea is caused by either infection or food intolerance, and eating a fruit and grain based diet does not help with these. If infection is the cause, then a sudden change in diet will do little to stem the disease, and if the problem is a food intolerance, then only avoiding that food, and waiting for the disturbance to pass will suffice.
Many clinicians claim that BRAT is unnecessarily restrictive, as with these sources. I'm inclined to agree. The last thing I'd want to do