I'm pretty sure that DL holds the back row of the plane for families who want to sit together. Maybe they'll hold two rows back there along with some middle seats.

Otherwise, if you buy a seat without a seat assignment, then you are at the will of the airline. If you are too cheap to pay $10 - 20 more for a seat assignment, that's not my problem.

I will only swap a seat if it their seat is similar or an improvement to what I have. Since, I'm usually in EC or the exit row, I will usually tell someone to swap with the other person.