Because it becomes harder to "hide" things - like, the provisions being bargained for, or politicians' actual convictions about particular measures. There are items which now get passed in omnibus bills, bargained for behind closed doors by leadership, which couldn't (whip votes as ye may) be passed in up-or-down votes on their own merits. Those are, in my opinion, corrupt bargains, and shouldn't happen. I like legislative horse-trading - it's an important part of the democratic process - but I'd like it to be open and above board.