Wow -- I don't like Air2s, but I have to admit that reading some of the things that have been written here makes ME wince.
I do think Dan has a good point about comfort without a regulator in the mouth, but I think that for two reasons -- one, I help teach classes and watch students, and two, I had the same issue myself after my OW class. If I HAD to remove my regulator, I was extremely anxious and desperate to put something back in my mouth immediately -- I would not have been comfortable (in fact, I WASN'T comfortable at the beginning) doing a primary donate, especially if I had had to fumble at all for a backup.
And, years after I discarded the Air2, I had an embarrassing and scary experience in my own swimming pool. My husband and I were practicing the gear exchange for my DM class. He uses a SeaQuest Balance with an Air2 for his pool setup, and I started out in his gear. We were buddy breathing, and as I went to take the equipment off, I lost my balance and fell over. He had the regulator at that point, and I believe I had my mask off. At any rate, I couldn't see where he had gone, and I was pretty CO2 loaded from having been buddy-breathing for a while, so I didn't have a lot of tolerance for not breathing. I went for the Air2, and, unable to see, I couldn't immediately figure it out, which added to my discomfort. Bottom line, I ended up close to panic in the bottom of my own pool, largely from CO2 retention. When we ended the exercise, I came up spluttering and saying, "I HATE that thing . . .!" Which is not a condemnation of the Air2, but definitely a story illustrating that its use may not be intuitive, if the person who owns it has not practiced with it.
I don't think anybody is going to be hurt by ascending vertically while sharing gas, although I think such ascents are harder to control and more task-loading of the divers. I'm quite sure it is safe and probably desirable to omit a safety stop when doing an air-sharing ascent with someone who does not practice such things regularly (and those people are very unlikely to run out of gas, anyway). I think worrying about the difference between vertical and horizontal decompression efficiency at the end of a no-stop dive in an air-sharing situation is rearranging the deck chairs on the Titanic. I think one can probably hook an elbow around an ascent line, if that's necessary. You can manage it all. I just don't really understand why anyone would WANT to. There are so many minor disadvantages of the setup, and only one minor advantage. It doesn't pencil out for me, but then again, diving is very much to each his own. I certainly wouldn't have any issues diving with someone who used one, so long as I had assurances that they had practiced with it.