We fly Air Tran most of the time, and I have always paid for the $6 seats, because, quite frankly, I don't really want to end up in the middle seat between two people I don't know. The usual configuration on the Airtran flights we take are 2-3. I had one instance where my son and I traveled together, and we payed for our seats, two seats on the side where there were only two seats. On that flight, Air tran changed the plane, and we ended up with three people in the row. The chick who was assigned the middle seat was already in the window seat when we got there. I told her that we had paid for our seats, and I would like to sit in the window seat, then I turned to my son and asked him if he wanted the middle seat or the aisle seat, and he chose the middle seat. So, she got an aisle seat.
My son actually prefers the aisle seat, since he is now 15 and nearly 6 ft tall, when we were travelling as a family of 3, I reserved two seats on one side of the plane, and the aisle seat directly across. My husband unwisely decided to go to the restroom immediately on boarding the plane, and some woman plopped her child into my husband's seat (her child was about 12 years old). I immediately told her that he was sitting in my husband's seat, and she went into a histrionic crying fit, telling me that she had missed her connection, or something like that (we were in Orlando, and not sure how much sense her story really made). All I wanted to know is really what seat her son had been assigned, because, had it been a middle seat, I would have insisted that he sit there, and not make my 6'1" husband sit there. Fortunately, it was an aisle seat, but, the attitude of the woman really made me want to ask for a refund of the money I had spent reserving the seat.
So, my point is that, if you take Air Tran, and don't pay for a seat, remember, other people have. Don't be rude and negative if they don't want to change. If somebody is nice and polite to me, and asks for something, it is much more likely that I will give it to them than if they take without asking. And if somebody does give you want you want (demand?), then say "thank you".