I've found numerous threads here on FT and elsewhere that say otherwise, including this one:

http://www.flyertalk.com/forum/marri...y-credits.html

That's also been my own personal experience. I've always received credit for Marriott timeshare exchanges. 14 days last year. 21 days so far this year (and 14 more planned).

I've found some threads since I posted my question about MVCI exchanges in relation to promotions, confirming that MVCI exchanges don't count towards promotions.