Soundtrack Pro has a great feature: "replace with ambient noise". This would be great to see on Audition, with a shortcut key. I use the command hundreds of times when editing dialogue recordings, replacing noisy breaths, coughs, etc where silencing the audio would be obvious.
There's a mix-paste in Audition, which is close-ish, but more cumbersome.
But my biggest thing on the wish-list is to be able to save MOVs with the video and audio intact - surely can't be difficult... I'd probably move entirely away from the Final Cut suite if it did that. I don't want to have to open video in Premiere and re-export just to normalise audio.
With Soundtrack pro (or at least with the old version I'm using for fear of the dreaded ear blasting bug in newer versions) it's: Drag film to icon, command-L, Enter, command-S. All done.
Writing to video formats, whlie not difficult is certainly not trivial and is a feature we plan to implement in a later release of Audition. I do hope we can make it as simple as you've described. I don't know if a feature for "ambient replacement" already exists in our database, but I'll take a look and if not, add a new request with a link to this thread. Thanks for the suggestion and support.