Wheatondiver, you have good eyes.
The theory behind the regulator on a necklace is the philosophy of "donate the primary". This means that, in the event that you need to provide air to someone else, you give them the regulator YOU are breathing. You know it's working, and working fine, and you know what kind of gas is coming through it, because you were just breathing it. There's no risk that it's been dragged through sand or coral, or isn't working because nobody's checked it in a while. The out of air diver gets a working reg. And you? You gave yours up -- But you have a backup regulator just beneath your chin. No fumbling for it, no trying to get it out of a securing device. Just pop it in your mouth, and you're good to go.
Of course, if you're going to donate that primary regulator, it's nice to have a little longer hose on it than the 24 or so inches that is standard. (Unless you love staring your dive buddy in the eyes
) A 36 or 40 inch hose will work -- the 40" is nice because you can route it under your right arm and then up to your mouth (although that may require a swivel to sit comfortably). Then you don't have a huge loop of unnecessary hose floating around your head. The 40" hose is just about standard octo length, and allows you to air-share and still have a little freedom of movement.
Even better is a 5' hose, which routes under your right arm, across your chest, around the back of your neck from left to right, and then to your mouth. This is nice because it's very quick to donate -- grasp the hose next to the regulator, and duck your head, and slip the hose over your hood and present the regulator to your buddy. Now, you can do a direct ascent, if that's the best strategy, or you can swim to an upline or exit, if you're in a place where a direct ascent might be awkward (eg. far from an anchored boat, or in a shipping lane).
A lot of technical divers use a 7' hose to allow air-sharing through a place where you have to proceed single file. But that works better with a technical rig, with a canister light or something to catch the hose at your right hip.
But the "donate the primary" strategy is, at least to me, a very good one, and I can tell you from personal experience that I have been glad on more than one occasion that my backup regulator was sitting under my chin where I didn't have to do a "sweep" to find it.