Lots of off road riders use them, particularly when they are carrying lots of gear. They are easier to strap on to and under things and they don't tend to bust when the bike goes down. They are also soft when you land on them (or lean on them) or when they hit you when you fall over.
They aren't difficult to secure at all. The decent ones have eyelets you can use to tie them down. Not all bike are big sheets of plastic with boxes everywhere like our ST's. Rolling them up when empty as Lionel does is a great advantage. They don't suck in and crease when empty and it gets colder. But as others have said, they can be more fiddly to use.
No additional safety issues I can think of when compared to the hard ones, just different benefits. The powers that be must agree, they've been selling them for quite a while now.
The rotopax plastics are good too, particularly if your bike/rack/topbox/panniers suit the lockable mounting brackets. If I were going to stick a jerry in a pannier or topbox they'd be the ones I'd use. Very solid and the tops sort of ratchet. I've not been able to make one leak fuel or fumes yet. However I generally don't put jerry's inside panniers etc. Not long back I rode with a very experienced LD rider who had one leak all through his pannier, yuck. Fortunately we weren't relying in it being full as part of the ride plan. But we are quite restricted for choice on the ST's.
Oh, reduced temperature --> lowering gas volume in a sealed container --> air pressure pushes in the jerry - they don't actually suck in......zzzzzzzz. I hope no-one noticed or if they did they read this far.
