I skimmed over this, so apologies if I missed something, but it seems that using Dropbox is a problem only when collaborating.
I’m writing a novel and I will be the only one touching the files, ever. (When it goes to edit, the editor will get a Word version and I will stop writing/editing at my end.)
So, for single-person use, is Dropbox sufficient for day-to-day writing?
There is actually very little difference between using it to collaborate and using it all by yourself. “Collaboration” just means more than one individual loads the .scriv file, and must follow all of the same rules you must follow, such as not opening it twice on different computers, allowing it to fully upload and download at the end and beginning of each session, etc. Collaboration just requires more communication to coordinate—which is of course nothing new. 
Having read all the way through this thread, I think we’re arguing in circles. Naive tools cannot solve issues of changes by multiple people. That’s the sort of issue source control systems were designed to handle. However, they are not really for naive users. Any source control system will ask the user to merge and resolve conflicts when there are changes made by another user.
Putting a scrivener project and all the rtf files etc into a distributed source system like GIT does work, since the files are almost all “text” files, and the merge process will show the individual “lines” to be merged. For RTF these will be paragraphs or control strings, and the problems will occur when there are changes to both versions of a paragraph or control string. The user will need to edit manually. And you need to avoid “binary” formats that source control tools usually cannot merge. At least with a distributed source control system, you can back track to find changes that collided and deleted someone’s work - each person’s delivery (in GIT a commit) is atomic. You can find the deleted text again, and re-apply it.
I don’t use dropbox, I do use GIT with GitHub, but then I’m usually the only person changing my scrivenings. I also work in the software industry, and use source control tools tdaily. If you are the only user, working from different location, then drop box should work fine.
I don’t think you can expect code to solve all your collaboration issues - if you are getting into the realm of “professionals” then you need to move from naive user to work and think like a professional project team.
For colaborations, you still need to impose discipline and work practices, like portioning out work. That way, only one person is changing a file or files. Break your colaboration into several scrivener projects, one per collaborator. Then one of you acts as project manager/librarian, and handles the merging of changes from contributors into one master.
Or export chapters to individuals, and then deal with importing their changes.
David 
Another way of doing this is to have everyone share a .scriv project on Dropbox and have a strict schedule on when you can open it; or an effectively way of communicating if it is currently open. Scrivener will try and protect you from opening an opened project, but its better to have a system. Then a method of using labels or some other obvious meta-data flag for “check-out” is important. You check out pieces of the outline and mark them with your name; drag them out of the project and into the binder of your offline working project. You work on them and when you are done, you drag the pieces back into the collaborative project and remove the check-out. Labels are pretty good for this, since you can tint icons and such with the colour. Give each person their own label and now you know when something is out, and who has it, so they if they forgot to check it back in or never got around to editing it, you can ask and clear it. It’s not strict, and it requires cooperation and communication, but it can work well.
There is one issue that perhaps Lee could address in the future. Suppose you and I are collaborating on a project, and we download the same .scriv project, and then each of us proceeds to create a new chapter. Scrivener’s node numbering scheme will assign each of our new chapters the same number - the next in sequence. When we try to sync or upload, there WILL be a clash. Scrivener could use some form of UUID for each node/file so that we both will have a unique string assigned. Until then I think collaborations really need to have seperate projects for each collaborator.
Your “checkout” method will work provided only one person can create new files.
UUIDs are something on the long-term plan for improvement/consideration; nothing firm on that, but it isn’t off the table last I checked. Back when the original project system was designed, Dropbox didn’t even exist and UUIDs were rarely used—syncing was something nobody outside of IT circles thought about. At this point it would take major redevelopment and carefully planning.
The checkout method works fine so long as only one person opens the project at once. Creating new chapter folders/files/whatever is something you would do in your personal offline project, and then drag them into the collaborative binder when you have it open. New IDs would be issued when that happens. Dragging between binders doesn’t retain the original IDs. To keep your offline version fresh, you could use the Save As feature when ever you are done. This would simultaneously give you a new copy with all updates, and close out the collaborative project.
Just got around to reading this, interesting stuff.
FWIW, I use DropBox as a backup for everything (nothing collaborative though). At the risk of being mildly techie:
I’ve got a file called copyscriv.bat which just has these three lines to delete my Dropbox backup folder, recreate it empty, and copy everything from my Scrivener docs folder (whic has my projects and backups) to it:
rd /s /q C:\Users\dmh\Dropbox\Scrivener
mkdir C:\Users\dmh\Dropbox\Scrivener
robocopy /E C:\Users\dmh\Documents\Personal\Writing\Scrivener C:\Users\dmh\Dropbox\Scrivener
I put copyscriv.bat into my Startup folder so it runs every morning when I turn my PC on, and I never have to worry about it 
It’s a long time since I’ve looked at this thread, but took a moment this evening, and I think this method from Ioa is the best of any heard.
Should really work for collaborations - or even for keeping track of what you’re doing as a single writer, if needed. It’s technically sound, and usable by anyone without needing to know that.
You just have to keep it in mind and use it…