From 1913 onward, voters have directly elected U.S. senators in statewide elections.
This change has led to a number of negative results, including
-Vastly increased federal power and vastly decreased state, local, and personal authority due to the state governments losing their representation in the federal government;
-The domination of Senate elections (and legislation) by forces outside of the particular states wherein elections are being held, e.g., out-of-state donations, political party operatives, and campaign consultants; and
-A decline of the influence of individual voters and small, local associations of voters over who is selected to be a senator from their state.
Individuals and small associations matter little to the statewide candidate but are important to the state representative and state senator who actually lives among them, knows them, and is known by them. The state legislator must take them and their views seriously, regarding Senate elections and other legislative matters, for they hold great electoral power over him. So individuals and small, local groups would grow more influential in U.S. Senate elections if the 17th were repealed, and outside interests less so. (I explain this in more detail here - JohnJ)
A crazy quilt of locally devised laws stretching across the United States may nauseate the federal bureaucrat who delights in the efficiency resulting from bland uniformity, but it would be pleasing to the citizens who would live under the aegis of those laws. Repealing the 17th would allow liberal, moderate, libertarian, and conservative communities to live under the laws of their own choosing rather than the choosing of the imperial few (of whatever political philosophy) in D.C.
Opposition to federalism is generally based on the same grounds as opposition to capitalism - that people cannot be trusted to make their own decisions and lead their own lives. But there is no freedom without choice, and that includes entrusting people with the responsibility to bear the burdens of their own choices.