I have not completed a scientific study, but I can share my experiences and reasons for choosing the $10/month "Enhanced" plan.
First and foremost for me was the ability to customize the layouts so that they looked less like parked sites or MFA sites. I like changing the layout design, colors, and using my own images. Additionally, I like the ability to use my Adsense if I so choose, add a You Tube Video if I want, add an RSS feed, and add affiliate banners to my pages. This site,
Compact Digital Video illustrates how I have used most of these options to make a more attractive (at least to me) page.
While I don't have hard numbers to back up my claims, I personally believe that when a visitor visits one of my pages, they are more apt to click an ad or a related site just because the site looks less "spammy". As I have said before, I am only using domains on WP that have no natural type-in traffic and perform poorly as traditional parked sites. I attribute everything they earn to WP, and they are earning significantly more than the $10/month I am paying.
Once again, my only regret is that I don't have more time to transfer over and customize more domains. I currently only have about 60 domains set up, but with time, could very easily have 200 or more. All for the same $10/month. To me, as long as they are earning back the fee, I think it is a bargain.