I guess you have never been mugged by a panicked diver who rips your reg from your mouth. I have and the first reaction is not to calmly deploy the long hose before going to your backup especially when they are either desperately trying to claw their way to the surface or sinking. In a normal air share with a diver low on air or having issues with a freeflow it may not matter as you have already deployed the long hose in advance but when the sht hits the fan every little thing matters.
So you didn't answer the question directly, but I interpret your response to mean that you are predicting that the air share emergency is going to be so "upsetting" that you will put the alternate in your mouth before you take the time to remove the long hose from your neck.
I have never used a long hose, never had one wrapped around my neck, but I have heard a million times that it is not a problem getting it off your neck and that it is a simple learned response of looking down and pulling the hose and it all deploys instantly and infallibly- just as long as you are not wearing a snorkel. Perhaps getting the long hose deployed is a bigger deal than I am lead to believe by the proponents of the "system"? I don't know and don't claim to know.
However, I do know that in a real air share emergency, my first concern and objective is to get a regulator into the other person's mouth. Getting a regulator in my mouth is NOT my first priority! This is based on my own experience, not guessing or supposition. I've had my reg removed from my mouth several times , against my will, and it has never caused me panic or an inordinate amount of fear, but it DOES one hundred percent, cause you to take immediate notice.
I can secure the air 2 breathing source about as quick as I can retrieve the necklaced second stage. There is no difference, and either way, it only takes 2-3 seconds, max. I can handle the reg out of my mouth for that amount of time. Is that (potential) delay, what is driving your position on this issue?
I have no problem with people NOT using an air 2, I am just want to explore this specific reason, because I have never heard of it before.