Holy cow! I've read through all this, and everyone has talked config, hose length, and everything else . . . but didn't address your question!
Disclaimer: Not an instructor.
Bottom line - in the real world, an out of air diver does not come up to you and politely report "OOA" and let you hand them a regulator.
You will be body slammed by a somewhat-panicked-to-fully-panicked diver clawing at your face for your air.
If YOU practice on the primary, you have practiced going to your backup over and over, so when your air disappears out of your mouth, you will (hopefully less stressed than the other diver) go to your backup.
SSI 2016 standards states that SSI advocates donating the primary as this method can be used with almost any regulator configuration (one of which is listed as the long hose configuration, which I personally use for my recreational diving and would advocate it to any certified thinking diver).
I don't understand this. Students are not trained to use these other regulator configurations, so why teach a method of air sharing that is (in my opinion) sub-optimal for the typical regulator configuration that most students are trained on?
Disclaimer: Not an instructor.
Bottom line - in the real world, an out of air diver does not come up to you and politely report "OOA" and let you hand them a regulator.
You will be body slammed by a somewhat-panicked-to-fully-panicked diver clawing at your face for your air.
If YOU practice on the primary, you have practiced going to your backup over and over, so when your air disappears out of your mouth, you will (hopefully less stressed than the other diver) go to your backup.