Hmmmm ... some of the info here doesn't agree with my experiences. I always use a bungeed backup, whether diving tech, rec, or sidemount. And how I fill my SMB depends on which one I'm using. For most dives, I'll use the small, 1-meter bag, in which case, oral inflate is easy and works well ... and as already pointed out, makes buoyancy control simple because you're not changing it as you inflate the bag. For some dives I'll want the bigger bag, either because I'm in more open area, or the wave height might make the smaller bag harder for the boat to see, or some other reason that makes me decide to deploy a bigger bag. My larger bag is about 2 meters in height, much wider than the small one, and is semi-closed. In that case, I use my primary reg to inflate it ... having switched to the backup before beginning the operation. I do it that way because the bag has about 40 lbs of lift, and I want to keep it well away from my body, reducing the potential for the line to snag something that would then drag me to the surface. Sometimes I'll be deploying this bag from pretty deep ... 130 feet or so ... on a deco schedule ... so I really don't want to run that risk.
I think there's way less chance of snag hazard while holding the reel/spool and line out at arm's length and using the primary reg to inflate it than there is by holding it in close enough to use your breath to inflate it.
And if you're worried about "wasting gas" to fill your bag, I'd posit that maybe you're cutting your gas reserves a bit too thin ...
... Bob (Grateful Diver)
. Although I don't use it, because oral inflate is so easy with the small bags I generally use.
