As a professional photo toner and retoucher, having a tablet is a choice. I've worked with other pros in the journalism field that swear by large tablets for toning and retouching, I personally don't use one mainly because the paper doesn't supply it and I was never trained on one.
Having used one for a short while, I can say that it does help out a lot once you get the hang of using the pen, and it's the most effective way to create a path around an image for extraction or selective adjustments.
It's not a must have by any means, but it is a benefit to have.