How is having the Project file in your DropBox difficult then caring and not forgetting a mobile drive?
On top of that as mention housekeeping?
Have the Events on the faster internal or Thunderbolt drives, on each Mac and have DropBox targeting the Final Cut Pro X folder.
Not saying your known alternative is bad but it sure is no more difficult.
I can see something like DropBox coming from Apple.. especially since Avid is pushing there online alternative.