Individually, it may be rational not to vote, but collectively if too many people have that attitude, democracy is meaningless and ineffective. In other words, voting is a collective action problem. The lazy approach is to be an observer of democracy. Simply echoing what pundits say the inevitable mainstream candidate stands for, and only paying attention to what will eventually be on the ballot. But, as a participant in the democratic process you have a responsibility to help shape what's being decided. Showing up isn't enough.
So, pay attention to what people are proposing (even fringe candidates) and don't hesitate to voice your agreement / disagreement to your representatives. If you don't, how are they supposed to know whether they're accurately representing their constituents? The privilege of democracy is the right to participate; taking this for granted is short-sighted and foolish.