> advertisers will just require that you compile their library into the first party js code, negating any benefit from such a security model.

It will become harder for advertisers to deny responsibility for ads that violate their stated policies if they have to submit the ads ahead of time. Also site operators will need a certain level of technical competence to do this.

The advantage would be that I know beforehand, and have the opportunity to test and, possibly, reject, what the advertiser want me to send to someone’s browser.