The pro is that you get more people's voices being heard.  The elections truly represent the will of the people because all the people vote.  This is in contrast to the US where a president might be elected even though only about 30% of those eligible voted for him.

The negative is that you get a lot of people voting just because they have to.  They don't care about the issues and they don't know about the candidates but they vote anyway.  This is not beneficial to a democracy.