The main advantage is that there is no change in buoyancy while you are prepping or filling the DSMB and you need only make one buoyancy adjustment during the entire process. These are distinct advantages over other methods and allow for a few precious seconds to ensure neither the DSMB nor the reel line are tangled in any gear, without ascending in the water column. For hunters who take their whole garage underwater with them, this is essential.I've been taught this method, but I find it increases complexity for no benefit. We don't hold on to our DSMB, once we have enough gas in it we let it go. Now you're being pulled up while your gear is pulling you down and you have to reestablish buoyancy and trim. It's just a terrible method IMO.
I like to keep things simple where that's possible. Prepare the DSMB for deployment and then use second stage exhaust gas, another reg or LPI to inflate the DSMB. It only takes a couple of breaths at depth and the buoyancy barely has time to change. Additionally, my trim doesn't suffer and I'm not vertical, kicking up, while trying to establish buoyancy again. So not only is it easier, faster and more convenient, but you'll look much cooler doing it
The only moment your buoyancy changes is when you release the DSMB, at which point a quick burst from the LP inflator will reestablish neutral buoyancy. You cannot maintain neutral buoyancy using the method you describe without first emptying some gas from your BCD prior to DSMB inflation, and then adding it back in after DSMB release (two buoyancy adjustments). The method I described above removes one of those buoyancy adjustments.
I respectfully disagree with you. With practice, this is the simplest, most efficient, and arguably the safest way to shoot an SMB from depth, IMO. But, everyone knows opinions are like a-holes....