Stu, I agree that it requires trust in your buddy’s abilities if he is using an integrated octo. The proper use of one requires practice to control buoyancy because it must be timed with breathing. That said I prefer it because of the streamlining and have my primary second stage on a long hose for the conveniences and safety it offers.
As for in and instructional environment I am a firm believer that it should be introduced during OW classes. My reasoning is simple...Whether you like them or not they are common place equipment in use by divers today and as such Instructors owe it to their students to make them ready to deal with a dive buddy who uses one and the differences during an OOA situation. Since buddy breathing on a single second stage is no longer a requirement of standards during OW the switch to a buddy’s primary second stage for accent should be covered. As an Instructor my students can expect this to be covered and discussion of why is within my mentoring. Any less isn’t doing students justice and furthers the idea of an integrated octo being an inferior product which IMHO it is not. It is a component that requires practice to develop skill in it’s use. I proved this to myself by using it during my IDC and IE and feel I am comfortable with my skills using it to be able to control my buoyancy while in a overhead obstruction environment.