What value is there in allowing politicians to keep their own statements of blatant self interest out of the public record?

I think that requiring politicians to make public their statements explaining their the decisions on behalf of their constituents would be and unalloyed public good.

Sure, this is fine, but that's very different from requiring all their interactions to be public. Put it this this: how often do you ever hear anything insightful or intelligent in a public congressional/senate hearing? It's always politicians trying to ask really stupid gotcha questions that will make them look good to the audience at home.

You attack someone, you better know you can be attacked.

I think this would work wonders to introduce principles into people that are anything but principled.

If you vote for someone that says he's for X, but then he actually secretly negotiating to drop X from the table, then what good was that campaign promise / vote ? Shouldn't it be necessary to know you were effectively defrauded ?

There's almost no instance where more transparency is a net loss to society. Politics is far and away one of least transparent undertakings. It is desperate for more transparency, not less