More people voting in the US → The political conversation shifts left as more progressive/leftists candidates are viable. A lot of elections in the US happen in small amount of votes, also known as narrow margins.
People not voting because not voting for the perfect candidate feels icky → As the conservative candidates are more favoured by people voting less, this shifts electoral politics right.
I would prefer to live in a world where Americans would vote between centre and left candidates, for instance, than centre and right (relatively speaking, of course) as they do today.
Remember when evil was allowed the greater evil to win (Reagen/Thatcher) and the Left in the English-speaking world turned more left as the after effect, creating a wave of new and innovative left-leaning thought?
Oh right, Bill Clinton and Tony Blair.