So, full disclosure, I'm the faculty advisor of the student chapter at my University, but I'll try to be as fair and balanced as I can.
What I tell the students is that, for them, membership can get them plugged into a professional network sooner, which can provide a lot of resources that can help make it easier for them to navigate their entry into the working world,
As an academic, it's a place to publish and, as such, they have a good collection of recent literature on technical communication from professional and academic realms. Membership provides access to this resource.That would be a way to keep up on things (as the articles are pretty contemporary) while you're between work.
However, as a professional, they've not done a good job in marketing their benefits, so it's value seems to depend more on the vitality of the chapter. They are pushing certification, but that hasn't broken out of the chicken-and-egg cycle (Certification has little value until it's recognized and it's not recognized unless it demonstrates some sort of value). I'm not convinced that certification is the future of tech writing, but I could still be convinced.
From what I've seen, if you're in an area with an active chapter, it could provide you access to people who can help grow your network. If your local chapter is not particularly active, or not particularly local, it might be harder to justify.