From my experience:
Agents are involved in selling books to publishers (a form of marketing the book), but they are not involved in marketing the book to the buying public, which is way different, way more labor intensive, expensive, and requires a different kind of expertise.
Agents are involved in helping with the interpretation and handling of contracts to best benefit the author--to maximize what the author receives from the publisher. If the agent takes over the duties of the publisher, that same contract expertise may work against the author, and represents a huge potential for a conflict of interest situation. At the very least, it erases the original benefit to the author.
Some agents will help an author with editing and re-writing a manuscript, but I doubt any legit agent will have the time to help with all of the line edits, and final formatting checks, the detailed evaluation of the page proofs, contracting with cover artists, doing all of the ISBN and copyrighting gruntwork, etc. It will only take a few of these clients to swamp the agent's workload.
Finally, if one goes with the average sales from self-publishing, 15% of very little isn't going to encourage the agents to put much effort into any of the above.
And a couple of gut feelings:
In the present agenting system, a significant number of books that are contracted by an agent do not sell to major publishers. In this case, the agent has to work hard to get a sale. What's to stop that agent from not doing that hard work, and rather just say, "Screw this. I'll have the author self-publish and I'll get 15% of what they make anyway."
If self-publishing is chosen, the author will still have to fork over a significant sum of money to have the book published, and now he/she will have 15% less of the profits to recoup that large initial expenditure.
At present, all inaccuracies, negative issues with the writing or the stories, legal issues, issues with quality control, etc. rest solely on the author's shoulders. If an agent takes a cut and performs some services as mentioned, all of those issues will be tied to that agent's name and reputation as well. Most important, the agent will not likely have a legal team to help with problems of story-based liability or problems with copyright infringement and intellectual property theft.
These are just a few concerns that come to mind.