Well, in the case of motorcycles, they are unsafe primarily for the driver, and not really for anyone else, so as long as the driver is aware of the danger they pose to themselves, there is no issue.
You can indeed, and I have done so, ever since the things became mandatory many years ago.
I wouldn't ever drive ten meters without my belt on, but I'm all for the freedom of anyone to be an idiot, as long as the idiocy isn't directed at anyone else.
The idiocy of not wearing a seatbelt is directed at everyone else. An unbelted driver is more likely to completely lose control of the vehicle after an initial collision and end up causing addition damage to others. Plus now that we effectively have socialized medicine we all end up paying for the idiots.
Although decapitation by windshield is unlikely (due to their design), you don't have to be going very fast to seriously damage yourself on the windshield if you're not wearing a seat belt.
I crashed a truck once, going maybe 30-40 km/h, wearing a seat belt. I ended up with some minor bruising from the seat belt, and no other damage, but everything else in the car went flying, including the glasses off my head. If I wasn't belted up, my head would've hit the windshield or steering wheel quite hard, and I likely would've needed to go to the hospital.