This is my perspective on it. There are 2 ways to do this.
1. If you can get a total picture lock on your project, Audition offers lots of things to take advantage of. Like a library of music/sfx as well as an abundant amount of nondestructive efx thru the racks.
2. I tend to send just clips to Audition because the projects I work on never really are totally locked. Therefore, fixing noise or adding certain effects that PrP may not have (though it does have a good selection) is my preference. That way if I need to lengthen or shorten a project its very easy to do. Trying to do that with a mix from Audition could be a time consuming PITA.
Tom Daigon
Avid DS / PrP / After Effects Editor
http://www.hdshotsandcuts.com
Mac Pro 3,1
8 core
10.6.8
Nvidia Quadro 4000
24 gigs ram
Maxx Digita / Areca 8tb. raid