THe whole point of the 17th was to allow politicians to reach around state governments and use mass media to appeal to the population. This was done under the assumption that the average person had better sensibilities for his self government. Frankly, I would agree with this were it not for the fact that information is controlled, distributed and manipulated by a relatively small number of people who control the media, and hence, the public opinion. In essence, the influence of the state governments was replaced by that of the media. There is, however, one balancing feature, and that is that much of the media is controlled by big business, and that in this fashion the intent of the founders is still, to some extent, alive in the current system. I agree though that the 17th should be trashed. Under the 17th, Senators could much more readily be recalled and replaced by their states.
As for the 13th, 14th and 15th, they should never have been necessary, as their intent was inherent in the constitution. However, they were absolutely necessary to fully implement the ideas of the founders in practice.
Also consider with the 17th Amendment, the Senate becomes redundant with the House of Representatives. The Senators are more powerful for having the same amount of power in the House concentrated in fewer hands.
It's a nation killer...through slow poison.