I have always paid for my seats on Airtran. I have always felt more comfortable sitting in the window seat, especially on the side where there are only two seats. I was in 19A both ways.
I watched and watched the seating as time went on - it seemed like the seat next to mine remained empty - both going down and coming back. No one had picked it.
Lo and behold, going back to BWI, a lady sat next to me and then asked if I would switch so she and her husband could sit together. At first, I said no way, I paid for my seating assignment, but then I asked her where he was.
He was in the last row of the plane, middle seat on the opposite side of the aisle. I was like, are you serious - I don't think so.
So don't arbitrarally expect someone to switch with you, especially if you didn't pay for your seating assignment.
Same thing happened to me last year, except the guy got on the plane and actually sat in my seat before my boarding zone was called. I had an aisle seat in like 13B (two seats on the left and three on the right). When I boarded there was a lady in the window seat and a man in my aisle seat. I politely told him he was in my seat and without even looking up from his book he told me, "I know! I want to sit by my wife, you can have my seat in 28E". Didn't even ask, just told me he was trading his middle seat in the back of the plane for my aisle seat toward the front. I told him in no uncertain terms, "I don't think so! Get out of my seat". He stood up and tried to intimidate me with his size at which point I stopped being polite. I told him to move or I'd get the FA to move him. Both he and his wife started calling me names and the FA saw them, came to find out what was wrong, then the idiots started the no English routine. I smiled and said that's odd, you were speaking perfect English just a minute ago when you called me a b-tch. The FA made the man move back to his seat and he started arguing with her, demanding that she
make me give up my seat for him. She told him he could either move back to his seat or get off the plane. After he finally moved to his seat, his wife/girlfriend started giving me a hard time. Finally after the flight took off and leveled off, I told her to either leave me alone or I'd be calling the FA to deal with her. She finally shut up, but continued to give me the evil eye.

Not that I cared. I put my earphones in and ignored her.

I couldn't believe that he called me a "rude b-tch" when he was the one who didn't even bother to ask me to trade seats, he just took mine and demanded me to go sit in his.
In the last year I've flown Air Tran in early May, late August, early Sept, mid Oct, and early Dec and on every trip it seems like I have seen parents and even just adults having a fit at the gate agent, demanding they move people around so they can sit together. The response has always been the same: if being able to sit together is important to you, then you should have purchased your seats. I hate to fly, hate middle seats, don't particularly like window seats, and will not sit in the back of the plane, so I'll pay the money to get my preferred aisle seat in the front of the plane. Even if I'm flying solo, I purchase my seats, so unless you have a better (or at least another aisle) seat to offer, then don't bother asking me to switch. If you do ask me to switch at least ask me politely, don't demand.