I say is worth it 50% of the time. I know, I know, I'm a lot of help!!
We've used Airtran four times. The first time we did not pay for seats, just asked at the gate and they put us all together. (At the time, we were two adults and two kids) The second time, we were two adults and four kids, ages 7, 4, 2 and an infant. When we saw we were ALL over the plane, we asked at the gate and they told us if we wanted to sit together we needed to pay for seats when we booked. They suggested we ask other passengers to move once we boarded. As I'm a bit shy, my husband did the asking. Turns out the best we could do was have an adult with the 4 year old at the front of the plane, an adult with the two year old and infant at the back of the plane, and the seven year old by himself in the middle seat two rows behind the adult at the back. Yikes. It wasn't good, and I hated the fact that we needed to do the asking. I hoped the flight attendent or gate person would have done it like they did the first time. So, now we always pay for seats. It KILLS me to have to pay for specific seats, but at least I know that all my kids will be with an adult.
So there, not paying for seats works about 50% of the time!! Once the kids are all older, like teenager aged, we'll skip paying for seats.