As Steve has said, you can't always keep up with the mutations: this year's vaccine is based on last year's main virus strains, so it will not and cannot protect you against any new mutations that have emerged in the ensuing year. Which explains why you can still get flu despite having been vaccinated. (and if you develop some horrible symptoms soon after vaccination where's the proof that it was the vaccine that caused it, or was it something that was going to happen anyway?)
I have had flu vaccinations for years - when I was younger, for occupational reasons - a) to stop me spreading it around my patients and b) in the hope that I would not have to let the NHS down by going off sick. Now that I am older, I have it because it's recommended for my age group, and have also had the pneumococcus vaccination (a one-off) so that I don't die of pneumonia.
I do recall one year being totally floored by flu for more than a week, despite having been vaccinated, but that would have been a new mutant strain.
And no vaccine is 100% (except maybe smallpox??) - viz my son's recent attack of mumps despite having been vaccinated as a child