Well, honestly it's completely up to you if you want one or not. Necessary? I don't think so for everyone. For me, yes. lol I use mine to have a sort of timeline/portfolio of everything I make. See, I used to keep a copy of each card I make in a box. Now I am trying to get away from that (trying to be earth and space friendly) and just keep pictures to remind myself of what I've made, and send the actual cards out. After all, that's what we do this for, right? And, I get to share what I love with others. What's great about blogs is that you get the chance to get feedback from others on what they think of your work. Some people like the interaction. I know I do.
Now as a demonstrator, you have the chance to share ideas with your customers, if they are checking your blog. If these are things that you are going to share at a workshop and you want to give them a sneak peek, great. Then it works for you. If you really want to promote your business, though, get a demonstrator website through SU!. Then your customers can order directly from you.
Now, when should you get one? Whenever you want! It doesn't matter if you aren't a demo, have been one forever, or are just starting out. If you want to blog, you blog. Really, it's supposed to be for YOU, not for THEM. If they enjoy it, that's a huge bonus, but it's not the only thing to do by any means.
As for how easy they are to set up, it's super easy. Go to
www.blogger.com and set up a blog for free. They walk you through the process of setting it up for yourself. It may look basic at first, but there's always more goodies to add as you go/learn. And, you have lots of people here who are willing to share what they know with you. You're never in it "alone."
I think blogs are FUN to have, and if you're interested, I say GO FOR IT. You'll never know if you don't try!
Good luck, sweetie~