For me the Expand Audio and being able to so very simply fade in and out and do overlaps and split edits whilst working on just one Track/Layer is absolutely brilliant.
In Legacy I would habitually be working across five video tracks or more "checkerboard" style in old dubbing parlance (with ten or more audio).
It was messy but the reason I did it was to be able to have all my audio there for cross fades and split edits as I tidied up later on. I saw a lot of other peoples' timeline and think many people worked that way.
Somebody brilliant looked at this and came up the most most mesmerisingly brilliant solution and alternative way of working.
Just a word of caution when working with expanded audio. Specifically, there is some unexpected behavior when you expand the audio to apply a video-only transition and then you make a cut at some point on either side of it. The in/out point of the audio is automatically moved to line up with the video transition and a fade in/out applied. It can end up leaving you with unwanted pops, lip smacks, breaths or any other sounds that you carefully trimmed your clip to NOT have. Totally unacceptable for an NLE to make changes to my in/out point without being instructed to do so. If it's a J/L cut you're fine, if you apply a transition to a clip with collapsed audio for an A&V transition you're fine, if you expand audio because you want a video only transition and the audio is a straight cut centered on the video transition... Beware. Be aware.
From another thread- http://forums.creativecow.net/thread/344/18364 -I was also having trouble with the unexpected audio fades. I investigated the strange behavior and posted a video showing the cause and a way to get around it (nudge).
On further investigation since then I've found that even if the audio is collapsed after applying a video only transition, making a cut will still move the audio in/out point and apply a fade. You just can't watch it appear, but you'll know when there's suddenly a little bit of a cough or something unwanted in the audio. Nudging to offset will still prevent it though.