Whats so good about them?
Those other bags shown are good, but as people have pointed out, there are alternatives, like the lomo ones which I have a few of. – I was only talking about the Monster bag which you can fit a whole bike in, you only take off front wheel, turn bars sideways, pop wheel in little wheel holder area, it zips up and has carrying straps. I have yet to find a good alternative, mine are still going after about 4 years. That’s the interest in duffbags for me. It would be good to see someone making an alternative bike sized lightweight waterproof bag.
There are chain reaction and tifosi bike bags, but they are padded, which for bus/train/air travel might be handy, they dont fold up nicely under the front seat of my car. You need a lot of room to store them. The duffbag is heavy nylon/cordura type stuff but folds up to an inch thick and about A3 size. \Plus the other bags require both wheels off and a further degree of dismantling. The duffbag is part of finishing my ride and putting my bike int he car, front wheel off, into bag, off to pub. It also disguises the fact its a bike.
I guess its horse for course, it very much suits my needs, and as yet nothing else does.