Act is too much of a crutch when it is working right.  Act will not force a student to fix a problem they have in flight like an instructor will.  The instructor can take the measure of a student and realize what the student is capable of even though the student might not see it in themselves.  I have had students I have been training get themselves in trouble about three mistakes high say to me instantly too take it back and stabilize it and I ahve said back too them you screwed it up fix it yourself I will not be here indefinitly to correct your problems.  Of course if they get very low I will take it back but that has gotten rarer since my ability to assess the students capability has improved.  Its truly amazing what a student will teach an instructor in the course of a lesson.  An instructor will force the student to learn faster then an inanimate object and it is not as much fun when there is no one to share it with.