We don't have a democracy in the United States. We have a representative republic. There is a big difference.

Not being involved in the political process just completely removes any influence you have on the process and the outcomes. However, if your belief is that not voting is the right thing to do, then by all means, please abstain from voting. I'd rather only have engaged people vote anyway.

EDIT/Clarification: We don't have a pure democracy in the United States. We have representative democracy and a republic as pointed out below.