I'm also just a minor contributor without much legal knowledge, but I'd probably be fine with it. Are there any big disadvantages we should know about? Would the code just be in the GNU repository, or would it also ship with Emacs? There's more factors in this decision than trying to gain new users, but if Evil replaced Viper in the vanilla Emacs distribution we could probably lure more Vim users into trying it.
Since Vile seems to be imposing itself as the most popular VI-like package in Emacs nowadays, it would make sense to include it in Emacs. But so far the main focus is on getting it into GNU ELPA. This said, the copyright rules we follow for GNU ELPA are specifically designed to make it easy to move code back and forth between Emacs and GNU ELPA, so once in GNU ELPA, moving the code into Emacs should be easy, if and when we want to do it. Stefan