I have always liked the idea of a swag and I keep checking out the different ones that come up in the hope that someone comes up with something clever for a motorbike.
The thing that I find is that I can get a 3 man tent for $50 or even less and they do the job well, I have room to sit up, and room to store plenty of stuff inside it. It's cheap, so if it's wet then it doesn't bother me folding it up wet ifI have to. I can even take a double air mattress if wanted and still store stuff beside it (hence the 3 man tent instead of the 2 man). Folded up it fits across the back of the seat and at $50 if it gets damaged or lost then it's not much money to replace it. The problem that others might find is whether they like air mattresses or not and there is the problem if it gets a leak. I find that a single air mattress can be put inside the lid of my top-box, or pannier, or molded around anything tied to the bike. I guess that there is no reason why you can't take self inflating mattress instead, but they tend to be expensive and there's a balance between cost and thickness and fold up size with them.
The possible advantage of the swag is the easy of setup and pull-down (which includes the mattress when doing it)!
I just thought that I better mention because they're the advantages and disadvantages that I have mulled over for a while.
But I do like the idea of throwin' a swag on the back of the ST and headin' off!