You don't NEED a lot of brewing related tools to make beer. You may WANT one or find it useful. Basically, a stir plate is going to provide you a more "set-and-forget" approach to building up your yeast. It makes it easier to rehydrate dry yeast and can produce a few more billion cells they shaking a starter every so often. I think its worth it, but I also spent $100 last year on a thermapen, so I may be biased on my reliance of gadgetry
If you haven't already, check out the book "Yeast" by Chris White and Jamil Zainisheff. Its a good read and can help with a lot of your yeast related questions.
As stated above, it's not a neccessitiy however it greatly helps. I built one out of left over junk I had sitting around. As a matter of fact I had a starter on it for the past 3 days. Lots of happy yeast! If you ever plan on banking and harvesting yeast or making a large starter then it's a huge help in the process.