The reason for that idea, for me, is that if the Senate's supposed to confirm appointments and laws, then they should bloody well be aware of the impacts of these things, not just whether this'll hurt or help their reelection chances.
True but that means when law and government because complex enough it just... turns into its own thing and becomes almost self feeding so that puts a wall that keeps everything the government is there to support out. Science, teaching, engineering and 3rd Class Telephone Sanitizing no longer have a voice in the decision making process, only the politics of them does.
In an ideal (perhaps impossibly ideal) world "Politician" should be a skill one has on top of an area of expertise, not one in and off itself.