What Fletcher said. Basic MMD is… really basic. It is no more taxing than composing a text-only email with emphasised words. The primary goal of Markdown (the syntax engine that MMD uses to generate its output), was to be easy to type in and easy to read without rendering it to some other format.
Integration with Scrivener is excellent. It automatically cuts up your document into a Binder outline when importing, and creates a simple single file output for your colleague to work on. Your workflow will look like this:
Export Draft from Scrivener, using “MultiMarkdown” export format. You don’t have to worry about chapter syntax, because Scrivener does all of the dirty work for you based on the Binder structure and document titles. This will make a plain text file suitable for external editing. When you receive the updated file: Delete the contents of your Draft, and with the selection on Draft in the Binder, go to the Import sub-menu, in the File menu, and select MultiMarkdown File. No more splitting up long Word documents!
All in all, I would say that if one is willing to learn a few simple text mark-ups, MMD is one of the better collaboration methods, short of two Scrivener users working together and using a version tracking system such as Subversion.
Now as for LaTeX, that is different. It is important to understand that MMD does not mean you have to use LaTeX. It happens to be the easiest way to make a flawless LaTeX file from scratch, but the export engine itself is quite flexible, and can produce any number of file types. LaTeX has quite a learning curve, especially if you have no experience with using UNIX software. The Mac has some pretty handy installers for LaTeX, but chances are you’ll have to get down and gritty with parts of the system that do not even show up in the Finder. It produces outstanding documents; beautiful to look at and easy to read–at the cost of some complexity.
Despite all of that, LaTeX is a very mature publishing platform with a large user base and truly extensive documentation. Getting answers using Google search is often quite simple, if you know roughly where to look. And, in many cases, things work right out of the box and you’ll only have to learn one command line: “pdflatex filename.tex”.
That was all my long answer. The short answer is: MMD is so basic, most people already know how to do everything they need to do in it, without even realising they know how. Because Scrivener has structural knowledge of an MMD file, importing and exporting work like a charm (as long as the structure is logical). LaTeX is decidedly advanced, and while it probably produces the best print quality you’ll get off of a Mac, it should be treated as the professional tool that it is. Fortunately MMD is all about choices, and if all you need is an RTF at the end of the day, it can do that too.