Then there's the anti-negative ad provision. Once in office, elected officials tend to do many bad things that upset people, like vote to raise taxes and then use federal money to build monuments to figures such as Dr. Seuss. Challengers, to be successful, need to point these things out. If you don't do that, you can't get the cheap ad rates the senators voted themselves. But Smith sees a much more self-interested motivation.
Negative ads are most effective against incumbents.
If you want to beat an incumbent, you first have to explain to people why the incumbent shouldn't be in office. The McCain-Feingold bill, some version of which is likely to make it through the House, is a good enough reason for me.