The other benefit I appreciated when I first joined was the big packet of reference materials they provided, which covered publishers, agents, and the business in general. (More of that info is now available on the internet, but it was very nice to have it all in one place.) The newsletter is pretty good. I found it to be the best value when I was querying and learning about querying, but now that I have an agent and a couple of books in the pipeline, I've still maintained my membership.
SCBWI also offers some grants (though you have to compete for them), and I think you have to be a member to apply for them? So if you're in the market for grants, that might be something else to look at.