The idea behind using those strains is that 1. We can't really do better, 2. Even though these strains are going to mutate before those 6 months, it is unlikely that all of the virus' proteins will mutate so drastically that all the antibodies we've made against the vaccine will be useless. After all, it only takes one good antibody for your body to be able to fight.

That being said, I agree that you don't need the vaccine if you have a fully functioning immune system. Idk elsewhere but in my country the vaccine is only recommended for people with weak immune system (elderly, auto immune disease etc).

The only reason you'd want to vaccinate even though you're not part of these groups of people is to protect the ones you know. That's why health workers have to get the vaccine, and you may want to get it if you're in contact with immuno-weak people (grandparents, newborn...). Not to protect yourself but to protect them.

Edit to add: some years, people who pick which strains to put in the vaccine simply get it wrong, we get a completely different strain in the winter and the vaccine is basically useless 😬 I remember that happened a few years ago