Single Thread or Multiple Simultaneous Projects?

I've often wondered if I manage novel development differently than most authors.

I write multiple novels at that same time, usually having a variety of books at various stages of development at any moment.  When I'm thinking about development, I usually break down the novel-writing process into the following steps:  Concept development, first draft, second draft, third draft, fourth draft (when needed), outside editing, proofreading, and publication.

Currently I have six novels that fall somewhere in these steps.  Specifically, two are in the proofreading stage, one is ready for a fourth draft, one is ready for a second draft, I'm working on a first draft right now, and one other novel awaiting first draft.  Typically I'll try to move the ball forward by a single stage on a project before letting it sit for a while as I'm working on other jobs.  For example, I'm writing the first draft of "Supply Chain" right now.  Next, I will work through the proofreading corrections on "Pursuing Other Opportunities," and then probably write the second draft of "Synergy."  Eventually, I'll come back to "Supply Chain" for a second draft, but it could be a good six months before that happens.

I developed this method on my own, and like it primarily because of what I call the "fermentation" factor.  By this, I mean that as the project sits in idle mode, I come back to think about it often, pondering character, plot, and scene elements of the story.  When I do pick the book up again to work on the next phase, it is usually with a fist full of ideas for story improvement.

Also, it keeps me from getting tired of the book.  Each time I pick up the novel, I start with excitement and enthusiasm, rather than feeling worn down by the number of times I must read and edit the project.

The biggest downside is the amount of chronological time it takes to go from start to finish.  With six projects in process at once, it can take several years.  And projects tend to clump.  Last year I definitely put more writing work in on the various projects than my single published work would imply.  This year, I expect to have three come out, and then the next year -- who knows?  It might be back to one again.

So how to other authors handle multiple projects?  Do they take one project from cradle to grave before moving to the next one, or do they use parallel paths like I do?