Only times I ever don't are when it's a very short journey and there's something preventing me, e.g. I'm holding something in the boot or the belt is stuck. Once or twice I've been in between people in a way which meant it was unlikely I'd be harmed too, but again, rather short, safe-ish journeys. I've been in a looot of minor accidents and general knocks which would have broken several parts of my face or worse if it weren't for that little locking mechanism. The occasional choking or whiplash I might get, or the 2 seconds-tops action every time, is a very minor set-back compared to reconstructive surgery/death.