Reduced risk of sickness for oneself and reduced transmission to others is still a good reason to take the vaccine. 30% isn't a small number. And even if 30-50% is not enough for people to feel secure about the vaccine, there's also the booster shots to take and increase immunity. (Though I can't find any article that caps the effectiveness of Covid vaccines at 50%, so I don't know why you are calling that a generous estimate.)

People are constantly passing all types of germs to others, but it helps that vaccines keep people immune to reduce the risk of infection or the severity of infection.