I also use Aperture along with iPhoto. I do all my processing in Aperture and then export the 5 star ratings to iPhoto. The reason for this is so that I can use the iPhoto's new features such as facial recognition and places tagging. It is also an easy way to convert raw files to .jpg by placing them in iPhoto. I also like the new slideshow presentations in iPhoto for viewing. Another reason I use iPhoto is the easy sync to my iPhone to store my favorite photos to take with me to show off my pictures.
Hope this helps
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