Not really true - the way they have to be ridden today though is different. I remember Chris Louis saying he'd not like to ride todays bikes as they only really suit slick or consistent tracks, full throttle all the way, as you'd ride in Poland but in years gone by more throttle control won you races and you could use grippy/slick parts of the track to your advantage more. Seats are also much higher these days. The issue with any fence is that you go from 60mph to zero in a short space of time so injuries will happen no matter what but we do see more use of the wide outside lines than ever before (where only the very brave/skilled would tread), which could be the feeling of safety brought about by air fences.