>So, what is the benefit of going through a third party vender rather than directly through Overture itself?
Accountability and experience!
Simple example. Let's say OT do your feed and you offer free delivery for your product. You may appear for the phrase "free product". With OT you have to pay for the click - with others you may not.
Your feed URLs may not have any content to build decent feed data and some vendors provide copywriters to describe the page content - OT does not. They just pull the data they are given and prepare it in a preset formula - not neccessarily in the best way.
Remember, some of us have been doing feeds for years - OT has been doing them for only a few months!
There is a very good reason why many people choose to use other vendors. Most people using feeds are fully aware they can go to Overture directly but many don't - anymore than they would use Overture to set up and maintain their PPC accounts