Smashwords is beyond great for getting your work into retailers you wouldn't otherwise be able to. (I'm looking at you Apple.) but I'm not sure using them to go through Amazon is something I would do.
Amazon makes it easy to get a well formatted book on the Kindle, and while Meatgrinder does a pretty good job, there are bound to be issues that are easily avoided with a direct upload.
I'm not yet so lazy that I'll sacrifice a couple hours work for a 15% cut in pay either.
But I still love Smashwords and the great work they do, I might be willing to give it a try and see what Meatgrinder does to my work. If it formats acceptably, well 15% of what I make isn't that big of a sacrifice to support a great organization.