I don't know that I can comment on the advantages of a separate project manager without knowing the precise use you have in mind. If there is an integrated program that does it well for you, then it seems unlikely that a separate program will have any advantages.
When I say project manager, I mean something that manages the files in a document, if you don't want to have a monolithic document. It also makes it easier for proofing. I did a 1500 word submission for one project, and still broke it up into multiple files- one for each scene in the document. Having to have one long document is very hard to work with in my experience. Not sure if most feel the same way. And having to deal with each of those documents separately is a pain also. I'd rather have one interface where I can go from document to document in the same session. It's the reason that I don't use vanilla word. After my add-in stopped working in the latest version of Word (http://writingoutliner.com/
), I tried to manage the documents in there, and Word lost a lot of work. If I'd been more patient, perhaps it would have come back. But I waited for 30 minutes, and it was still hung. And it did it several times. I found more lift in editing in chunks in Markdown, outputting to one HTML or PDF document, then pasting it into Word for the final formatting. And then there's the problem of formats... my client decided to have us submit over google docs, and google docs added in extra spaces, and completely messed up Word formatting, when it states that it imports Word documents. Now I'm just rambling.
But yeah, having something that has references to all of the documents that make up a larger document that can be opened and edited independently has become key to me, since I started writing semi-professionally.