My rationale has always been that if they save the plastics in a car park drop (or, not that I have ever done it of course, at the top of the ramp onto the ribbed steel deck of a cross-channel ferry
) then they have paid for themselves and I have heard of many, many more such drops than I have heard of folks throwing it down the road at speed.
The only person I know who threw it down the road and it didn't hit anything solid, was Costas. His bungs saved him a fortune! Normally a bike will hit something very solid after a slide on the road.
Others I know? One Old Git in Spain (and that doesn't count cos he threw it into a ditch which made a mess of a lot more than the plastics). Rae who bent it big-time, probably still upright in a hedge. A mate who rode into a motorway ditch in Belgium needed a lot more than crash bungs and it appeared that the bike remained 'upright' anyway!
If it goes down the road at speed then my dealer has told me of two cases of K12/13's where the thing looked OK'ish, but closer inspection revealed that the frame had bent caused by the R&G bung on the side!
You have to look to your own abilities and tendencies. Cases definitely help save the plastics in a drop but if you are prone to dropping the bike, and I have dropped quite a few whilst stationary'ish, then I would suggest the R&G type route.
If you are prone to chucking it down the road at speed I would highly recommend some training, better tyres and a few track days
Brian (who has an excellent sence of balance but finds most bikes do not
)