As a former GSoC mentor, I think it's important to have an onboarding pipeline in your project, and disagree with the notion that the mentor/student relationship is somehow atypical - such relationships pop up all the time in the natural course of projects, and are key to the health of most of them. It's up to you to select the students who you think will stick around. Given that, taking the time to onboard junior people is a really rewarding investment in the project.
(My student ended up going on to work for Red Hat. I don't presume I had a lot to do with it, but I think the culture did.)
(My student ended up going on to work for Red Hat. I don't presume I had a lot to do with it, but I think the culture did.)