One of the big benefits of using virtualenv is that you can change the environment your code is running within. For example, if I want to test a new version of a package (or a new version of python altogether), I can create another virtual environment and "workon [newenvname]", ./manage.py runserver..., and test away.
It also follows the principal of least surprise. As a developer, if I run a python file, I generally expect it to use the python version in my environment, not some hard coded path. This is exactly why manage.py references python in the environment (/usr/bin/_env_) and not a direct path to the python executable.
It's easy to switch virtual environments and unexpected that I'd have to change code.
