I've been using variable-pitch-mode in my Org files for a few years: I find it nicer for writing design documents, meeting notes, and documentation. And I use it for all Org files, as if I embed code in a source block, that's still monospaced and font locked. Best of all worlds :).

The only issue I have is that I use org-indent-mode, and sometimes the spacing doesn't 100% line up, especially with bulleted lists. But I have long since gotten used to that.

I use it in all buffers whose major mode is derived from text-mode.