Scrivener's great right up until the moment when you need to collaborate with someone else, at which point it falls over completely. The usual practice in these scenarios is to write in Scrivener and then when finished export the document to Word format to send to these other parties, but then if they come back to you with edits/feedback/etc. you're stuck with the problem of getting those changes merged back into your original Scrivener document, which is no fun -- especially if you've got multiple rounds of revisions among multiple collaborators to deal with.

Don't get me wrong, I like Scrivener a lot, but in lots of scenarios it just doesn't work as a drop-in replacement for a more general word processor.