SEO
On the topic of SEO, there are typically three ways that I have seen licensed content be syndicated to retain the SEO primacy of the original content creator’s version:
1. Using a noindex directive so search engines don’t index the syndicated copy.
2. Using a JavaScript embed so that search engines cannot read the syndicated content.
3. Using a cannonical directive to point search engines to the original version.

Based on what you described, you are saying search engines can reliable discern which copy is syndicated. Are you using a cannonical directive? Regardless, if the search engine is able to tell which version should be credited from an SEO perspective, then it’s not really helping the syndicated version.

Privacy
As I mentioned licensed content usually requires a pixel tracker. It’s great that you don’t force this on people who license your content.

Commentary
That’s a good point that you can add your commentary around the licensed content. It’s quite different than having the ability to intersperse your commentary. Commenting around the licensed content is akin to making a mix tape, where as interspersing your commentary is akin to making a remix. They both have different merits, but ultimately being able to intersperse your own commentary allows more creative freedoms.

Cost
I think you mean “I also have to ask whether unlicensed content is really free. Isn’t someone paid to create it?” rather than uncurated content. Unlicensed content doesn’t have to come from a paid publication, it could be created by a hobbyist blogger for example. Regardless, even if you curate content from sources where the original authors have been paid, the cost of excerpting or linking to the content in an ethical manner is free as compared to licensing the full content outright. For more best practices on ethical curation, see: http://www.contentcurationmarketing.com/content-curation-copyright-ethics-fair-use/