Let me just say that everyone who voted primary donate is wrong.
Ok. Just kidding. I'm still trying to wrap my head around the benefits though of primary donate and why the lopsidedness.
Here's how I see it:
Primary donate:
Step 1. Donate Primary
Step 2. Put alternate in mouth
Alternate donate:
Step 1. Donate Alternate
So one huge important step not needed that doesn't remove one's own gas.
Now I've never witnessed an OOA situation so I can't speak from experience, which is why I am curious. But there seems to be a lot of anecdotal evidence of divers who have had an OOA diver making a beeline for the reg that's in their mouth. Now it would seem to me, that a nice, bright yellow, UNUSED air source sitting right on a diver's chest is going to be a nicer target for someone about to suck salt water. But I've never been there, so I just don't know.
I'm one of the folks who voted primary, and I'm not a tec diver, but I have had a (buddy) OOA situation, and that was a large part of the impetus for me to make the switch.
It was my second dive of the day and my 30th dive ever. I was diving from a boat off Catalina with an instabuddy with a comparable level of training and experience. We both had rental gear; it was my first day trying out the BP/W I eventually bought, and I had my octo hose looped and stuffed though my left shoulder D-ring. Previously I'd been stuffing the hose into my BC pocket, as my rental gear didn't always come with retainer clips or loops and they didn't seem to work very well anyway. We were ascending from a max depth of 75 feet, already within touching distance and face-to-face, when she gave me the OOA signal. I was surprised because I'd just checked her gauge a few seconds before and it had shown 1,000 psi. She immediately grabbed my octo, before I even had a chance to react, but the hose wasn't sliding out of the D-ring very easily. I don't know how long it took us to get it out--seconds? Fractions of a second?-- but it seemed like an eternity; by the time we did, I was already taking out my primary to give to her so she could breathe while I worked on freeing the octo.
I told this story [eta: in Near Misses at the time it happened] to get some perspective on what might have caused the incident, but I also kept thinking about what I could've done better, and how it could've gone differently. If she weren't already within reach of my octo when she'd been unable to draw breath, and had signaled while swimming toward me, I like to think I would've been quicker on the draw and pulled it out for her. But then, if she'd been farther away when it happened, or just more prone to panic, she might've just ripped the primary out of my mouth in desperation. This particular instabuddy handled the situation calmly and correctly, but on a more recent boat trip, I narrowly escaped being paired with someone who ran out of air, forgot her signals, and blamed her buddy for not reading her mind and taking care of her. I often dive with instabuddies, or with friends who have even less experience than I do or who haven't been diving in a long time, so I need to be prepared for a wide range of responses to an emergency. I always do safety checks and make sure they know where my alternate is before splashing in, but that doesn't solve the problem of all the devices and methods I've encountered for keeping octos in place either insufficient (octos work themselves free) or too effective (octos don't let go easily enough when you need them to.) For anyone who's figured it out, good for you. For me, primary donate on a long hose with a bungeed alternate feels good. I've practiced handing off my primary and switching to my octo underwater, and I do it on land before every dive to show my buddy. I haven't had another OOA situation but I feel much more confident about how I'll handle it if it happens again.