For advertising outside of Facebook, I still think paying for good demo-data may be worth it, especially if you’ve previously validated it in some manner (though the cheaper the better). I still don’t know how some vendors get their demo data and/or how accurate it is, exactly. Just because there are more data options for a specific category of data doesn’t mean any of them are actually good sources/accurate/can scale.
Still a bit on the fence about DMP impression passbacks. It’s very, very easy to steal and repurpose audience data, even without a DMP (can you say DSP impression retargeting?) Ideally, I’d want to make sure no one else has data “equally as cool” as a vendor. Then I’d be confident about pushing back on this.