The advantage of storing documents in a DEVONthink database is that the text information of a wide variety of filetypes can be integrated into the information resources available within the database, including searching, See Also, etc.
Scrivener project don’t take well to being directly saved into DEVONthink.
But you can Index an existing project into DEVONthink. Text of your documents will be visible within the database. The information contained in your project files is searchable, integrated with the other information in your databases.
Want to open your project and work with it under Scrivener? You can do that from within DEVONthink by selecting a project file and clicking on the Launch Externally (Scrivener) button in your Toolbar. After saving changes within Scrivener, click on the corresponding project within your database and choose File > Synchronize to update changes into the database.
Many other applications such as Pages, Word, Excel, Numbers, etc. will allow you to save newly created files directly into your Global Inbox, by selecting the ‘Inbox’ destination under "Places’ in the left column of a Finder window. From the Global Inbox you can use the Move To command to file such new content into any location among your open databases. To edit such a document under its parent application, select it and click on the Open Externally button in your Toolbox. Save the edited file afterwards (File > Save) and the modified document is immediately available in your database.
The ability to unify the information content of a variety of filetypes and to view and work on documents in such a working environment is what attracted me to DEVONthink, way back in 2002. DEVONthink has gotten better and more powerful since then.
That brings me to a discussion of Full Screen, which I often use for viewing long documents (especially on my laptop), but which I never use for writing. (The default green on black view of text documents reminds me of DOS [shudder]. I set Full Screen to display text as black on white.)
Disclaimer: I’m not one of the developers, and I’ve never attempted to recommend anything about Full Screen to them. 
I do my draft writing inside a database, in rich text. I like the working environment because it puts the information content and DEVONthink’s tools at my fingertips. I can fish for ideas from within my own writing by invoking See Also, and perhaps add some interesting references as tabs to my rich text window. Or select two or three paragraphs I’ve just written, Control-click and choose See Related Text, to fish for ideas if I’ve hit a writer’s block. Or take a look at how others have used a term by Option-clicking on it. Or select an interesting phrase, Control-click and choose Search Selected Phrase. Or locate, with a quick search, other notes and references that I can create links to, perhaps for citation and footnote purposes.
I love that rich working environment when I’m writing. That’s why I don’t use Full Screen, which isolates me from all those tools. Some writers prefer Full Screen’s isolation as a means of preserving focus on what they are doing. But I don’t have a problem keeping focussed — as my dogs well know, when they are trying to remind me that it’s time to feed them.
There’s no accounting for personal preferences and foibles (including my own eccentricities). DEVONthink can accommodate a wide range of them. Some users, for example, prefer writing in Full Screen in typewriter mode. That’s easy. Create a document using an application that allows that, then import it into a DT Pro/Office database. Next, Export it as a Template. Choose that template (Data > New With Template), click on Open Externally and away you go! You are isolated from the universe and typing away in the middle of a never-ending virtual sheet of paper. Save your work, and there it is in the database. As for me, I’m 78 years old and spent way too many decades hammering away at typewriters. I prefer not to be reminded how clumsy and limited they were. Remember Liquid Paper?
Off-topic question: Anyone remember who wrote this, and which character spoke this line? “Sir! Your VIctorian vocabulary is a stigma symbol of outmoded infantalistic paternalism.”
) hasn’t proved to be a priority or core competence for DT’s maker. On the other hand, a strength of DT is that it is relatively easy to open documents in external editors and have the external changes visable via Quicklook in DT. Using the external editor gives access to the full-screen features that other posters mentioned, while keeping the document under management of DT. That’s a workable compromise, maybe?
So here’s to hoping for some improvement soon…