On IRC we get a steady stream of confused users (in both the #emacs and #clojure channels) who added melpa because some library recommended it in their readme and they didn't realize it would cause every package they use to also pull from the unstable branch. It is a huge support headache.

However, these days I only very rarely find packages that aren't available on Marmalade. When I do, it's usually enough to ask the maintainer nicely and they start publishing to Marmalade; it's not a difficult process by any means.