The problem with that argument is that it requires voters to trust their representatives' competence and motivations to represent their best interests behind closed doors. That trust is in really short supply right now.
I could make an argument that it is transparency itself that is helping to erode that distrust, as each side seizes on (and/or misrepresents) details to portray the other side as corrupt. When in fact most legislative fights are simply translations of differences of opinion into the legislative realm.
Accusations of corruption are a common framing argument; if I can get you to believe that my opponent is corrupting the system, I can make you believe almost anything about the issue or their point of view. This is much harder to do if both sides negotiate privately and then jointly present a compromise.
