Most of the time seat belts will save your life...but there are times that they won't. About a year before i met my husband he was in a very serious crash...him and a friend were in a little MG and they hit a truck the size of an F150. The MG split in two and they were both thrown out (the friend lost a leg and still isnt right). But if they'd had seat belts on they would both be dead. But he always wears them now. Most accidents people will have though will not be a major collision, and seat belts will keep you from dying or getting hurt very bad.
I've got an old MG that didn't even have lap belts. It was easier to install 5-point belts than 3-point, so that's what I put in. I've never checked it out but I've been told a couple of times they're not legal for street cars, but I'll gladly pay a fine if it ever comes to that. And continue to use them because in that tiny little car I think my chances are better with them if anything ever happens.