I'm finishing the front of my first t-shirt quilt and I'm wondering if using a thin batting is really needed or is it just personal preference?

Depends on how it will be used. I use a thin batting or an old blanket. I think you'll need something so your quilting will show up. I knew my hubby's would be hung so I just used a very lightweight cotton, gave it some body but wasn't heavy.

they just some times get heavy

I'm snowed in right now and had to search my stash.
I have some 100% cotton batting, and also some quilting flannel. I think I'm going to try the flannel (preshrunk already).
I guess it really doesn't make a lot of difference. They're both cotton.

I usually use the warm & natural for my T-shirt quilts. I did make three rag t-shirt quilts a couple of year ago where sandwiched the t-shirt then flannel then lightweight denim. These were a very nice weight.

I just finished quilting a T-shirt quilt for a customer. We used Warm and White. It is something like 80x90. I don't think it is too heavy.