I can understand why an OW instructor would have students learn and practice using the conventional rig, mostly because that is what they are likely to rent before they buy their own. I myself think the necklace system is the best. It is very much the fastest. With a little practice, the donor can have the regulator out and heading for the OOA dive in about a second after seeing the OOA signal. It is also the most reliable. As an instructor, one of the biggest problems I faced teaching with the conventional setup was the number of times the students' alternates had come out of their holders and were not easily available. I learned to make sure all alternates were actually in places and ready to go before I started an OOA drill.
When I first started tech training years ago, I used the necklace for tech but did not bother switching to it for my NDL diving. That changed after I read about a woman who drowned after going OOA. She went to her buddy for his alternate, but it had come loose from its holder and was stuck somewhere behind him. She panicked when they couldn't find it.