You need to either be an AP holder or a Florida resident in order to purchase the TIW card.
I am upgrading my 7-day ticket to an AP and we are getting the TIW card. Originally, we were going to utilize the basic
DDP, but then we had several more TS restaurants than the plan allowed, and realized we would have leftover CS credits too. I did the math both ways (DDP + extra TS cost vs. upgrade to AP + TIW card + 20% off the bills at TS restaurants) and it would basically be the same cost to us to do this either way. So we decided to go with the TIW card since it also covers alcoholic beverages, we can use the discount for the 4 meals we are eating with my parents, sister & nieces (there will be 10 of us total, I will be present, and the card is good for up to a party of 10...I will pay the bill and they will reimburse me, it's all worked out already!), the card is also good at lounges in the deluxe resorts (we're staying at BWV), and we can truly order what we want. Although we tend to be dessert people (which is why we liked the DDP when we used it in 2008, and why we were going to use it again), sometimes DH and I just share a dessert, or we can do something like have lunch at 50s Prime Time, and then later on get a big cupcake from Starring Rolls to share!
Keep in mind one thing...as I said, it was basically going to cost us the same thing either way when I crunched the numbers...but one thing also factors into this...we are DVC members so we get $100 off the cost of an AP. That made a difference too.