A quick read of the thread gives me two facts:
1. In the past, when proper equipment was harder to source, some divers used a plastic bag inside of a mesh bag to function as a lift bag.
In the modern world, no one should use an inadequate method such as this for lift. It is dangerous and impractical. A diver should use an SMB or a lift bag, which will have the proper attachment points for lift, as well as methods to control the rate of lift and vent excess air. It will also pose almost no threat to wildlife. I saw tens of companies offering quality lift bags and SMB's at DEMA last year, finding one should be incredibly easy.
2. DandyDon carries one to use as a signaling source at night. This is again, a poor method. As I have already explained, a proper SMB will be much more reliable than a trashbag. Trashbags have no method to vent excess air and as such, might be rendered ineffective by a diver who inflates them too much. Further, they might easily be ripped or torn on a dive. Additionally, they are not nearly as easy to manage and fill with air as a proper SMB. Finally, they do not allow for nearly the same visibility as a proper SMB. The light will be more diffused if the light is inside the bag, than an SMB lit from a light held on the outside of the SMB. The SMB will reflect more of the light horizontal from the water, as the trashbag will transmit light around it's globular shape. A light placed on top of a proper SMB which should be atleast 6 feet tall, will be seen in waves as high as 6 feet while an inflated trashbag will probably be no more than one or two feet above the surface, rendering it useless in anything but light seas. And, trashbags do not typically have a way to mount a light on the top. A proper SMB should have a way to mount a light on top, as well as the ability to shine lights on the sides, to create maximum visibility.
As such, I see absolutely no reason for a diver to carry a trashbag in the water.
Atleast there is a practical reason for a snorkel. I think even spare air is more useful than a trashbag, though I must concede, a trashbag of sufficient size would hold more air, especially if filled at depth from a freeflowing octo. However, a diver would need a ton of lead weight to hold them from a rapid ascent as they breathed from the TrashAir, so I'm not sure if the increased offering of air is as useful as it might seem at first.
If SMB's were available in a nearly translucent white, I would endorse the carrying of one at night, as it might then be a good altnerative to the classic red/orange/yellow SMB.
[edit] I don't want to come off as too harsh. Most of the time, I don't care how people dive. I am sure that DandyDon is careful with his trash bags. However, i want to discourage people from using trashbags because I feel they are less effective than SMB's by some small amount, and because I feel they pose a danger to sea turtles. After dating a girl who devoted two summers to sea turtle research, I have great respect for those majestic beauties of the sea, and I care about preventing the trash that kills them from appearing in the ocean. Because I feel most divers are less careful than DandyDon, I feel that encouraging the use of trash bags in the water is a bad idea.
Hopefully you all can see I do have a sense of humour and some things are said tongue in cheek. Even bad ideas are a good source for jokes
