When I was pregnant with our second child, my husband and I and our then toddler took a trip to Italy. We had no problems getting seats together on our outgoing flights, and we booked everything far in advance, but coming back we were unable to book seats together (I don't know why; we tried a couple of times waaaay in advance to get it fixed). We had two seats and a single one faaaaaar away. Our parenting strategy was always to work to keep our toddler entertained, taking shifts, and I was at a stage of pregnancy where I had to go to the bathroom frequently, and it would have been a hardship for us to have not been seated together, but one we would have dealt with if we had to without complaining but wanted to avoid if possible.
When we checked in at the airport, we asked the gate assistants for help, but they claimed they couldn't do anything and to ask the flight attendants. As it turns out, there was a block of 3 seats which were empty, as apparently the people missed their flight. A woman saw that block and zipped into it, lying down, as soon as the fasten seatbelts sign went off (it is easy to out maneuver a pregnant woman with a toddler, so she scored the move faster than I did). My husband asked her sweetly if she'd mind changing with me & our toddler, so he could help me with our toddler and she'd still get 2 seats (mind you, she'd only paid for 1). She refused because she said she had a bad back and needed to lie down. She was traveling with her husband but didn't care to sit with him (and also didn't want to let him share her newly-acquired bounty of space).
The FA (whom we'd asked on boarding if she could, if possible, help us find seats together) went up to this woman, fixed her with a steely gaze, and informed her that she WOULD be changing seats with me and my toddler. The woman caved at that point. She ended up still getting more seats than she'd paid for, and I was really bemused by her attitude that she deserved 3 seats to herself. I was so grateful to that FA for fixing things, and working in turns my husband and I kept our toddler amused & quiet (I couldn't have done that myself, particularly as I was at the stage of pregnancy where I had to go to the bathroom a lot). We got a lot of compliments after the flight on what a joy it was to fly with a well-behaved toddler. It definitely would have been a worse flight for many of us on that plane if that woman had managed to keep the 3 seats instead of settling for just scoring one bonus seat.
Like a prior poster, I always paid for the extra seat when I flew with a baby, and I always brought a carseat (I am very safety minded), and I always got hassled as it was presumed I had no right to that "extra" seat and was a special snowflake for having brought on a carseat.