After recently getting two vintage bagged machines (both Electrolux and Aerus), that I love and am obsessed with a bit, I actually thought about why I've preferred bagless machines for the past twenty or thirty years.

I realized that while storing/displaying my two bagged tanks upright on their end without the hose attached; that the self closing mechanism (that on type c bags is basically a cardboard piece that when the bag is removed presses against the rubber gasket part of the bag where it attaches) is not engaged at all when the bag is in the machine and you can see directly into your vacuumed debris which is only a few inches below the opening/manifold to the machine. 

The debris may or may not begin to have a smell, usually associated with vacuumed dead skin or dirt that was food particles, and will decompose over the week or two it normally takes a standard vacuum user (not any us, of course ) , to fill a bag before replacing it when mostly full after a few vacuuming sessions... and that opening becomes a potpouree into your closet or room unless you put in a fresh bag each use even after just a light vacuuming, which for most people would be cost prohibitive and moreover wasteful.

I think that the amount of twists and turns the air takes to get into the dust bin of a bagless machine from the manifold creates almost the equivalent of a p-trap (In plumbing, a trap is a device which has a shape that uses a bending path to capture water to prevent sewer gases from entering buildings, while allowing waste to pass through. In oil refineries, traps are used to prevent hydrocarbons and other dangerous gases from escaping outside through drains. In domestic applications, traps are typically U, S, Q, or J-shaped pipe located below or within a plumbing fixture.) effect and mitigates the amount of smell emited from a bagless machine, not totally, but more than most bagged machines stored without their hose attached, which in that case would act as a longer barrier for the gasses to escape into a room and create a vacuum aromatherapy effect!  Also, I think most users of bagless machines dump their debris each use, since it prepares the machine for the next use.

I also think that it was because of the push toward being "green" (and having less disposable pieces like bags) that was attractive to new consumers who were looking to jump on board that train (yes, they were not aware of filter cleaning efforts required - and most likely don't maintain their machines well, which is why they burn out or malfunction and they just get a new one.) and feel good about the lack of recycling and landfill they create from that one small purchase.

There could be a handful of other reasons as well for bagless, like seeing what you are accumulating each vacuum session, so you get that immediate gratification of "wow the machine is working, or...oh look, there's my earring I just vacuumed up"...etc.

I have now come to the conclusion, despite the slight smell, that I love having the classic design of vintage machines decorating my abode more then the wanting to avoid that slight smell coming from them while on display.