It took me awhile to get used to it, but there's really no reason not to use virtualenv.

The more you use it, the more you have to use it. I had two projects with different authentication mechanisms that were unfortunately named the same thing. I accidentally deployed one without virtualenv and spent the next whole day figuring out what happened.

Virtualenv is a life-saver, and it's so easy to do that you might as well just always do it everywhere.

Just hit a similar situation at work and realised why I had to use it. Had a django app called taxonomy in a project, couldn't use the same name in another django project. Now I definitely have to use it!