to be quite honest with you, i would wear a seatbelt even if it wasnt compulsory, like many of you have said its plain stupidity not to. i only wear lap belts in the middle seat of the back, they do provide protection, but be prepared for some seriously painful ribs and stomach problems if involved in an accident.
as for bikes, again i would make sure to wear a helmet and leathers, for the exact reason posted by ozzy, generally i dont like going out on a push bike without at least jeans and a sweater, and i have seen first hand what can happen, me and friend riding down hill going 30mph, as he went by me he hit a rock, and went flying, stripped his knees and arms of a good few layers of skin and he broke his wrist and dislocated his shoulder, had he not been wearing a helmet and trousers it would have been much worse.