I've seen mentioned in various places that it's important to belong to organizations like SCBWI and to mention it in your cover letter/query. How do you feel about that?There are many lovely things about the SCBWI. I've been a member myself.
The overarching thing that's great about the SCBWI, though, is that it gives everyone who already knows their way around the industry a place to send newbies instead of trying to give the uninitiated several mountain-sized clues oneself.
Membership in the SCBWI indicates a willingness to learn, and that absolutely counts for... a tiny little bit. Because it doesn't indicate an ability to learn, or an ability to write.
Willingness to learn is important, sure, because it shows you know there is something to learn.
But it really just puts you on the step above the dreadful amateurs who think they'll send the story they wrote for their grandchildren (the single story they've ever written) to a publisher and see what happens; the crazies who feel the world owes them a publisher for having written something (the single something they've ever written) of such genius and power; the maniacal jerks who think the only thing that stands between them and wild success is "persistence," defined as "pressuring, harassing, and stalking publisher staff."
Joining the SCBWI is a good thing, and if used well, it can be a great thing. But it's not going to the school of SCBWI that will get you published. It's graduating.