The diagonal cut is the only way to ensure the halves are exactly equal without doing any measuring (without getting into any mathematical hair-splitting, that is)! That way, when you share with a friend, there's no possibility of getting the "short end". :^)
I never learned to do french braids. Nor did my mother. In second grade, my idea of a perfect life was french braids, sandwiches cut diagonally rather than straight across, and snowball cakes in my lunch. Alas, these things were cruelly withheld from me. Sob.