The usual argument concerns the way in which power is allocated between the federal and state governments. It is thought by many that if Senators are chosen by and answerable to the state legislatures (rather than voters of states), then the Senators will be more reluctant to grant to the federal government powers historically exercised by state governments. To the extent that increases in the power of the federal government are attributable to mere power grabs as opposed to being a reflection of changing realities and changing perceptions of realities, the argument is probably a legitimate one.
smartly done
I remember the first time someone mentioned his desire to repeal the 17th amendment. Of course, at the time I was a "moderate" and had no idea what he was talking about. I got a copy of the Constitution, read it, and asked why this was a problem. The answer, similar to yours, contributed to my desire to embark on a course of study that took me from John Locke to Alan Keyes.