I'm sure this will be added in the future, but it is currently a dealbreaker for me - I'm using a single package that's just straight hosted in a private git repo, as the package itself isn't ready to be published yet. I'm sure other people are using private packages for more normal reasons (closed source, pre-production, etc).
If yarn actually made it simpler to refer to a local package during development, I'd be on board with that. (I am developing the dependency, but want to also work on the thing that depends on it, so I'd rather just save locally and refresh npm on the outer project, but that's hard to get right - file urls don't always update, the easiest workflow seems to be delete the package from node_modules, and npm install, with the package having a git url rather than a version in package.json).