It might be worth highlighting the limitations here in a sidebar.  This won't work if you need to modify C or Cython code in astropy, and it won't work with Python 3, since we rely on 2to3 to convert code from Python 2 to 3 (though the latter may be resolved in the long term).  There is also a class of corner case bugs that can be missed this way (such as data files not being installed), since you end up running code that is subtley different from what would be installed by `setup.py install`.

I generally recommend `virtualenv` for these reasons, and in the long run more involved developers should move in that direction.  It's really just a pedagogical question of how soon you want to get people off their crutches ;)