Personally I find it pretty painful creating large documents in Word. Creating documents in Latex helps me focus on the text/structure without worrying about figure/table placement (I can still have them in the markup, but and fix all the formatting issues after I’ve written the text).

I think it depends on author mentality somewhat. Word works well for some, Latex for others.

It might also be worth considering Markdown or plain HTML. Particularly if an ePub/kindle version is a possibility.