Specialists in public opinion have exhaustively documented the average voter's shocking ignorance about the main issues of the day, the names of their local candidates for office, or the policies the candidates support.
The flakiest voters -- the ones least motivated to show up at the polls year in and year out -- also tend to be most poorly informed. So when turnout drops, it tends to leave the pool of remaining voters with an improved average level of political knowledge and policy know-how. If well-informed voters have a better picture of the candidate or party most likely to promote the general welfare, then especially high turnout can actually tilt an election away from the better choice, leaving everyone a bit worse off. And that's not very civic-minded.