I have a 6' Halcyon closed circuit SMB that lives in a pocket until I am on the surface. If I need to be seen in 4' seas its nice to have. And I can impale my 100 lb open lift bag on it to increase the visible area up high.
The closed circuit design is nice in that it will not deflate at the surface, can be waved, etc. However, launching a closed circuit bag or SMB underwater strikes me as being extremely stupid and the practice launches I have done with one supports that view.
It makes little sense to have to disconnect an inflator hose from drysuit or wing just to launch an SMB. Adding 4" LP inflator hose to a stage or deco bottle works better, but then you have that hose along for the whole dive on every dive and it further adds to an already uneccesarily complex system. Simple is always better. The failure possibilities of the closed circuit SMB are interesting as a slightly corroded and sticky QD fitting could leave you getting drug to the surface by your LP hose - a great way to get seriously bent or dead.
From depth, a semi closed circuit lift bag or SMB makes much more sense. It can be filled from a second stage or from the wing/BC inflator (the inflator, not the inflator hose) through the open bottom with no requirement to attach anything to anything else and the butterfly valve inside ensures it will not dump its contents on the surface. It gives you the best of both worlds with none of the risks of a closed circuit SMB.
Also, once on the surface, unless you keep a couple pounds of negative bouyancy on the line, a tall SMB tends to lay down on the surface, defeating the purpose of having a tall SMB. A lift bag is never as tall, but it is consistent even when there is no download on the line. In the event you feel you need more visibility (drifting deco in 3-4 ft seas), you can run the SMB up on the same line as your bag using a locking carabiner clipped to the line.
So...in short I carry a pair 100 lb semi-closed circuit lift bags on the back plate, one yellow and one orange, and the 6' closed circuit SMB in a pocket for deployment once on the surface if needed and If I leave one at home it is the SMB.