When is it appropriate to use SQL Server Developer Edition? Do you use SQL Server Developer Edition on server-class machines in DEV and STAGING environments?

I am working on a large project where (if it passes the proof of concept stage) we will have several large geographically distributed enterprise class database servers running SQL Server 2012 Enterprise Edition.

We will have a production environment will initially have 3 servers, our Staging environment will have a minimum of 3 servers, and our Development environment will have 1 server (hosting three instances).

I was under the impression that we would only need to acquire Enterprise licences for the actual production servers, and we could get by with developer edition in our developer and staging environments because they are not "production".

Other sources have told me that we need to have an enterprise licence on all of the above mentioned machines, and that the developer edition is only meant for a single developer on their workstation.

Since developer edition has all the sexy features of Enterprise, I can't really see the value of it on a workstation class machine, especially for developing and testing the type of high availability system we are building.

If we have to fork out Enterprise licences for a dev server, that will just about kill our proof of concept stage, thus killing the project.  Forcing an enterprise licence on a staging environment will make management just want to skip staging altogether.
Development edition licenses can be used to install anything that isn't production, provided that everyone that connects to the server has their own development license.  MSDN licenses can be used as well if everyone has an MSDN license.