Writing it tail-recursively forces you to switch from the O(2^n) algorithm (which has two recursive calls; you can't put both into tail position) to a more effi

Writing it tail-recursively forces you to switch from the O(2^n) algorithm (which has two recursive calls; you can't put both into tail position) to a more efficient algorithm. You could also write the more-efficient algorithm without tail recursion, of course.