you should upgrade your battery but a cap is not a bad thing to have. batteries aren't going to do anything at all until your voltage drops below 13.. if you get a nice alt and a good cap your battery should not see load at all... you really need to understand capacitance in a circuit before you can say it doesn't do anything. One thing people fail to consider is that an alternator produces a ripple voltage because its using 6 diodes to convert AC to DC... capacitance helps to stabilize this, creating a smoother signal by filling in the gaps..
there is a lot of disagreement on this topic but there's my 2 cents
=p