Because it is more likely to kill you or your buddy than other alternatives.
In the event of an emergency when you need to use the Air2, you are also likely to have a situation where your buoyancy is also compromised. You are likely to need to make a controlled direct ascent to the surface with two divers in close proximity, who cannot be further apart than the length of the hose connecting them. Having the regulator on the same part of your BC as the buoyancy controls is going to cause you problems.
Imagine this situation: You're PADI trained, so in the event of an out of air emergency (OOA) you will give your primary reg to your OOA buddy, put your Air2 in your mouth, grab your buddy's BC, and then you'll try to make a controlled ascent. If you've already started open water training, you know how little space there is between you and your buddy when you're in this position; hoses are flopping around, your arms are in the space, you're trying to make room for each other to kick, etc.
What can go wrong?
1) Either you or your buddy may knock the Air2 reg out of your mouth because your BC inflator hose, which you need to have in one hand to control your ascent, is right in the middle of your 'area of limited space' (because the Air2 reg on the end of it is in your mouth). That can turn one OOA into two OOA real fast.
2) You might knock your buddy's reg out of their mouth, because you're gonna have your arm, your inflator hose, and your Air2 all right about mouth level in the 'area of limited space', and the chances that you'll give your buddy an elbow to the face are quite high. For obvious reasons, that's a bad thing.
3) You've combined two safety devices into one unit, which means you've compromised your redundancy. If your BC inflator hose fails, you lose both your BC, and your backup reg. If some element of the Air2 system fails and freeflows, you're going to have a rush of bubbles right at the same point where you'll need to grip your BC inflator controls -- that rush of bubbles will make the hose whip around and make it hard to grab and control.
Most "standard" rigs have a hose for the primary, and a slightly longer hose for the "octopus" or secondary reg. Ideally, in an emergency, the OOA buddy gets the Octo, and you keep breathing your primary. That's better than an Air2 (at least you haven't compromised redundancy, and you can get some distance between your hands & mouth).
There is a third option, which uses a "long hose". Search in the DIR forum for a lot more detail, but the summary is this: You have a long hose (5 to 7 feet) which you wear wrapped around your body to control the hose when in normal use. You also wear a backup regulator on a bungee necklace. In an OOA situation, you donate the long hose to your buddy, and you switch to the necklaced backup. There are manifold advantages to this system -- most notably, you and your buddy will be able to separate by quite some distance after the reg exchange, which allows you to both control your buoyancy without having to hold on to one-another, making the safe ascent following the reg donation much, much safer.
Ryan
