There are a few cases where an adult in good health absolutely should vaccinate : for example, if you are in contact with risk population (elderly, children, if you work in the medical field); you should also consider it if you are indirectly in contact with these populations (see : herd immunity)... and most everyone is, one way or another (a neighbour is a nurse, your colleague has children, ...).  
Some strands of flu also hit populations in good health harder, although they are quite unfrequent; it called a cytokine release syndrome (H5N1 recently, spanish flu a century ago).