Hi Michelle, First off congrats on your upcoming wedding. :-)
I was married in Cabo about three years ago, It's beautiful there. My thoughts on a WC is this. For me, we choose to do a legal wedding in Mexico and that proved VERY hard to get all the required paperwork and such on our own, so we decided to hire a WC for our very small wedding (10 guest) But more then that, she was helpful in securing the remaining vendors and making sure everything was taken care of, so it was worth the extra money.
In regard to a TA, well I say why not? We have some amazing travel agents on the forum that can work with your resort in possibly getting a group discount, and of course assist you incase something goes wrong. I have "Tammy host" do all my travel arrangements and I couldn't be happier, I recently returned from St. Thomas and the airlines lost my bag with some important photography equipment in it. I called Tammy and she started working with me and the airlines, so that I didn't spend all day on the phone when I needed to be at a wedding. So finial though, she was awesome. :-)
Best of luck and happy planning