This doesn't consider the use case of needing to have multiple versions of the same package installed for testing. Sure, it would be nice to be able to have a s

This doesn't consider the use case of needing to have multiple versions of the same package installed for testing. Sure, it would be nice to be able to have a second machine provisioned to do testing on. And, as a extension of that, if you are using virtualenv+pip for testing, it makes sense for the deployment to use that too, as that means that the testing environment is closer to the deployment environment.