I would suggest that the 17th Amendment (well explained in the first post), was/is a victim of the "Law of Unintended Consequences" as demonstrated in the 2008 elections, particularly the election of Harry Reid. Senate elections, once the province of individual states, have now gone "national," not because citizens of states vote for Senators but because money is poured into states to elect senators who are backed not so as to best represent the interests of the states but because the candidate maintains the dominance of one party or the other in the Senate. Again, the election of Harry Reid in 2008 is a good example of this. At least when the legislatures/business selected the Senators, there was a good chance that they would look after the interests of the state or a portion of it.
Ironically, in seeking to empower the voters of a state by this amendment, they may have taken power away from these citizens.
The 17th Amendment was a reflection of Progressive ideals because it took power away from big business elites and gave to the hands of the middle class. The idea of giving power to the middle class, which was seen by Progressives as the group that was most qualified to govern, was one of the major ideals of the Progressive movement.
Before this amendment, Senators were chosen by state legislators rather than by a vote of the people. This meant that people could be elected to the Senate without any real popular support. Big business interests would often "buy" Senators during this time. The Senators would do what the business interests wanted because they did not have to answer to the voters.
The 17th Amendment changed this by making Senators accountable to the people. This put power in the hands of the middle class.